Harman Blennerhassett was born October 8, 1764 to Conway Blennerhassett, a wealthy Irish land owner, and his wife Elizabeth Lacy. At the time of his birth, the family was not residing at "Castle Conway," as the Blennerhassett estate was known, but rather at the English village, Hambledon, in County Hampshire. The Blennerhassetts were away from Ireland to avoid the violent raids on prominent Irish landlords by a group of peasant outlaws known as the Whiteboys.

Harmon was a direct descendant of King Edward III and recieved a very thorough education at the Westminster School in England, where he manifested a taste for classical studies and the sciences. He entered Trinity College in Dublin and graduated with distinguished honors. Blennerhassett then read law at the King's Inn Courts and was admitted to the bar in 1790.

Like most young aristocrats of the day, Harmon undertook a grand tour of Europe after his studies. Upon his return, he nominally assumed a law practice. But, being heir to a large fortune, he was by no means solicitous in obtaining clients. He devoted his time to the cultivation of his interests in the sciences, music and classical literature. He was also, ironically, dabbling in revolutionary politics.

Whether or not he formed political opinions on his own, or naively followed the rhetoric of others, Harmon joined the Society of United Irishmen in 1794. This group, inspired by the words of Thomas Paine and the French Revolution, were dedicated to ending British control of Ireland. He must have had some leadership capabilities, or perhaps it was because of his fortune, for he quickly became the secretary for the group.

Although the third son of his father, Harman inherited "Castle Conway," when his older brothers preceded their father in death. He inherited a fortune of 100 hundred thousand dollars.  Already a practicing attorney, the future of this 27 year old seemed as bright as the morning sun.  

In that year, his family sent Harmon to France on a rescue mission. The Reign of Terror was at its height and the Blennerhassett family feared for a young relative, Margaret Agnew, who had been studying abroad.  Harmon immediately fell in love with his eighteen year old niece Margaret.

Mrs. Blennerhassett was in all respects a very accomplished lady. Her figure, tall and commanding, was moulded in the most perfect proportions. Her features over which was spread a most brilliant complexion were beautiful. A strong grace which intelligence alone can confer. Brown hair, profuse and glossy, dark blue eyes, and manners both winning and graceful, ever attracted attention to her even in the most brilliant circles…Her mind was as highly cultivated as her person. She was a recognized poet, an accomplished Italian and French scholar and one of the finest readers imaginable. She especially excelled in the plays of Shakespeare, which she rehearsed with all the taste and spirit of a first-rate actor. In history and the English classics she was equally well read, and was often called to decide disputed points in literature. Few women ever lived who combined so many accomplishments and personal attractions.

Harmon was willing to risk the wrath of family, society and Church.   To the shock and consternation of their family they married. The marriage was considered incestuous and immoral by the laws of both the Churches of England and Ireland. Margaret was disowned by the family and the couple was ostracized.

The  Irish uprising failed miserably in 1797 and led to the persecution and collapse of the Society of United Irishmen. Hurt and angered by his family and society's view of his marriage and fearing arrest by British authorities, Blennerhassett and his young bride decided to start life anew in America. He sold his estates far below face value for cash to establish himself in the New World. Before leaving he purchased a large library of classical and scientific books, along with equipment to outfit a laboratory to conduct experiments.

The couple arrived in New York City, where their wealth and attractive qualities drew them into that city's elite circle. They spent several months there making inquiries as to where they might settle. On advice from friends they decided to explore the Ohio River Valley. They spent a considerable time in Pittsburgh and then traveling down the Ohio on a keelboat, they saw and fell in love with Backus Island, two miles south of present day Parkersburg, West Virginia.

They purchased the upper end of the island and named their wilderness Eden "Isle de Beau Pre."

He began construction, which would take 2 ½ years on what was to become the grandest building west of the Appalachians. While building was going on the family lived in an abandoned blockhouse on the island left over from the Indian Wars.

The house and grounds cost an amazing, for the day, forty to fifty thousand dollars to construct. It was a strange mixture of elegant Palladian architecture, Irish country house comfort, with a look of Italianate Queen Anne villas of the nineteenth century. Having approximately 8,200 square feet of living space, the house must have been one of the largest in America.

A contemporary, Dr. S.P. Hildreth describes the house :

The island mansion was built with taste and beauty. No expense was spared in its construction that could add to its usefulness or splendor. It consisted of a main building, fifty-two feet in length, thirty in width, and two stories high. Porticos, forty feet in length, in the form of wings, projected in front, connected with offices, presenting each a face of twenty-six feet, and twenty feet in depth, uniting them with the main building, forming the half of an ellipse, and making in the whole a front of one hundred and four feet. The lefthand office was occupied for the servant's hall, and the right for the library, philosophical apparatus and study. A handsome lawn of several acres occupied the front ground, while an extended opening was made through the forest trees on the head of the island, affording a view of the river for several miles above, and bringing the mansion into the notice of descending boats. Nicely graveled walks, with a carriage way, led from the house to the pillars. A fine hedge of native hawthorn bordered the right side of the avenue to the house, while back of it lay the flower-garden, of about two acres, enclosed with neat palings, to which were trained gooseberry bushes, peaches and other varieties of fruitbearing trees, in the manner of wall fruits.

The Blennerhassetts filled the mansion with the finest furniture money could buy from the eastern United States and England. These all had to be hauled over the mountains and floated down the Ohio. Fine paintings, sculptures, oriental rugs, alabaster lamps, and marble clocks adorned the rooms. One room was paneled in solid black walnut. The gardens and greenhouse grew a variety of exotic crops. As stated earlier, Blennerhassett spared no expense in establishing his Eden.

John S. C. Abbott wrote of the man and his residence in 1875.

Thus there arose, as by magic, amidst the wilds of the Ohio, one of the most elegant mansions of modern days. All its internal appliances and external surroundings were of the most luxurious character. Mr. Blennerhassett's library contained a large and choice selection of the most valuable books. With native powers of a high order, trained by an accomplished university education, by foreign travel, and by intercourse with the most cultivated men of his day, he well knew how to use that library for his constant profit and for his unceasing delight.

Skilled also in the sciences, and with a strong taste for chemical studies, and all the correlative branches of natural philosophy, such as astronomy, botany, electricity and galvanism, he had supplied his laboratory extensively with the best apparatus for observation and experimentation which the arts could furnish.

He was constantly making experiments and eliciting new facts in these wonderful branches of natural science. In addition to these scientific accomplishments, he had made such attainments in the classics, that it was said he could repeat the whole of Homer's Iliad in the original Greek. In manners, Mr. Blennerhassett was very courteous, mild and yielding. His virtues were of the amiable character rather than of the more stubborn. He was easily duped by the intriguing who had sufficient sagacity to discern his weak points. His benevolence was unbounded, and his sympathy with the sick and suffering very intense.

Besides his books and scientific interests, both Blennerhassetts enjoyed hunting. Quail and other small game abounded on the island. The couple also enjoyed giving lavish parties for the local gentry and  blossomed as the premier social attraction of the Ohio Valley.  They used their island paradise to draw guests by the thousands. Blennerhassett and his wife, accomplished musicians, would play at these soirees. In this remote Eden, the Blennerhassetts had established a retreat from the objections of the outside world. Harmon Blennerhassett had it seemed, the perfect life of an Enlightenment country squire.

Due to the extravagant nature of the Blennerhassetts, accounts of their riches were greatly exaggerated. They had little income other than the interest earned by their capital which did not come close to meeting their overwhelming  expenses.

In the spring of 1805 the Blennerhassetts received a visit from the former Vice-President Aaron Burr. It is probable that tales of Blennerhassett's fortune had reached Burr who was in need of an investor for his plans of forming a western empire. The easily duped Blennerhassett was convinced by the witty and persuasive Burr to take part in the scheme. Margaret was completely captivated by Burr and his plan. Here was a chance, she felt for her husband to utilize all of his talents.

Blennerhassett's island would be the base camp and his money would outfit the expedition. He contracted to have fifteen large riverboats built to transport the yet- to- be mustered army  of  five hundred. One of these craft was a keelboat sixty feet long. This was to be the Blennerhassett's personal conveyance. As per his style, it was elegantly appointed, even having a fireplace and glass windows. Blennerhassett's money was also used to purchase arms, ammunition, provisions and whisky for a force of five hundred men.

Burr was only able to recruit one hundred men, for rumors were flying throughout the country concerning his motives. Burr was betrayed by co-conspiritor James Wilkerson to President Thomas Jefferson who issued a  proclamation calling for "all residents of the United States to bring to punishment all persons engaged in such treasonable enterprises as Burr's expedition".

Following the Proclamation, Governor Edward Tiffin of Ohio dispatched the state's militia to the Ohio River to block all traffic. The United States government also sent the Virginia militia to seize Blennerhassett and his island. 

 Harman  fled his island paradise only hours before the militia laid siege to it. Harmon escaped to Kentucky. Margaret was away in Marietta.

When she returned home, she found the house had been ransacked and greatly damaged by the militia.

Burr and Blennerhassett were apprehended and imprisoned in the Virginia State Penitentiary. Burr stood trial for high treason.The conspiritors argued two main points. First, no act of treason had ever occurred. Since the definition of treason in the Constitution requires an overt act of war against the country and since no act of war was committed then no act of treason existed. Second, arguing that since Burr was not even present when the supposed act of treason took place, he clearly could not be guilty.   Burr  was acquitted when a five month trial failed to produce any concrete physical evidence as proof of his plots. Blennerhassett's release soon followed, and both men were granted freedom, although their reputations, as well as their fortunes had been destroyed by the ordeal. Aaron Burr sought refuge abroad, in Europe.

Blennerhassett briefly returned to his island Eden, but could not afford to repair the damage that had been inflicted by the Virginia militia. The mansion burned in 1811.

Harman purchased a small cotton plantation and moved his family to the Mississippi Territory.  But, the Embargo, and crop failures doomed this venture.

 Destitute the couple returned to Ireland to live off the charity of an older sister. Harmann was able to find work as a lawyer in 1899 in Montreal, Canada.  In 1822 he returned to Ireland.   His wife who had considerable literary talent published The Deserted Isle (1822) and The Widow of the Rock and Other Poems (1824).

Harman died on the Isle of Guernsey  on February 2, 1831 from a series of apoplectic strokes.

After her husband's death Margaret returned to the United States. She petitioned the government for compensation for the destruction of her house. She asked for her rights not charity. Robert Emmet, of Ireland, and Henry Clay supported her cause. Congress decided to redress the grievance, but it was too late. Margaret  died indigent at a home for the poor in New York City in 1842.

In all, the Blennerhassetts conceived five children although they adopted another. Two sons and a daughter were born on Blennerhassett Island but only the sons survived infancy. Once settled onto their Mississippi plantation, Margaret  bore two more children, a son and a daughter. The daughter died in infancy.

Of the Blennerhassett sons only the youngest bore children, none of which reached adulthood, and when he himself passed while serving as a confederate soldier1862, the Blennerhassett name died with him. Another son died from a night of drinking and another died starving in a New York attic.

The body of Margaret Blennerhassett was exhumed in 1996 and returned to her home.  Harmon's body lies in an unknown unmarked grave on the Isle of Guernsey in the English Channel.