Castle Hassett, later Crevenish Castle, was built on the shores of Lower Lough Erne by Thomas Blennerhasset as part of the Plantation of Ulster.

The Castle was English in design - a three storey lime and stone rectangular house with tall bays on the corners, acting as flankers. In 1611, Blennerhassett had a joiner, a carpenter and three other workmen on site with him. Despite initial difficulties in attracting English settlers, Blennerhasset had succeeded in establishing a small village, with seven English families. The Estate passed first to Thomas, the eldest son and then in 1627 to the younger son, Leonard.

Leonard died in 1639. He was survived by his widow, Deborah, to whom he bequeathed one third of his lands and right of residence in Castlehasset. 

Deborah married one Captain Rory Maguire, who in 1641, was to play a prominent role as leader in the Irish rebellion. Captain Rory Maguire, by marrying Deborah, became the owner of Crevenish Castle.

Rory Maguire led the Irish Rebellion planned for 23 October 1641. Prior to that, he had invited Sir William Cole and other Protestant gentry to dinner at Crevenish Castle, intending to imprison them.  However, Cole had already been warned of the planned revolt by Brian Maguire of Tempo and before the dinner ended, he and the others got word of Maguire's plans to capture them and all succeeded in escaping to warn the others of the danger ahead.

Image of Crevenish Castle ruins taken by Vera Watters, 1980