It was built in 1185 A.D., by Liam Cassidy. It became the home of his decedents along with the Leprechauns of Cassidy Keep. It has been occupied continuously by the same family until the time of Oliver Cromwell drove them from residence. In 1659, the Cassidy family through a distant relation took possession of the keep, changing the name.
The history of the Cassidy family is recorded in the Great Hall, where portraits of generations of the family tell their own story of Ireland's stormy history. Many additions and alterations have been made but the contours of the surrounding lands have changed little in 800 years, retaining a sense of the past. The hall of the castle is of Norman architecture, the circular towers flanking the facade were added in 1765.
At the heart of the medieval castle is the Oak Room, approached by a winding stone staircase and lit by Gothic windows added in 1820 when the room was enlarged and the front hall below was created. The room is lined with carved oak from floor to ceiling, representing scriptural subjects, now black with age and polishing. Some of the carving is of Flemish origin, including six panels representing biblical scenes opposite the window.
Cassidy Keep changed hands when the current lord of the Keep died, leaving the family residence not to Tom, his son, but to his nephew, Sean Cassidy. Sean allowed Tom Cassidy to remain and Tom expressed no bitterness over the situation. However, years later Tom would tell Terry that Sean had won the Keep from him in a poker game.
Juggernaut and Black Tom Cassidy kidnapped Lilandra for Erik the Red but the X-Men track them down. A fight results in a stalemate until Gladiator, guard to the imperial throne, shows up to take Lilandra captive. Phoenix rescues her and vanquishes Gladiator for the meantime.
Banshee and Generation X traveled back to Cassidy Keep. They were met by Eamon O'Donnell who serves as seneschal (keeper) of Sean’s family home, Cassidy Keep. After helping the Leprechauns and the Fairies in Otherworld they returned to Cassidy Keep. However on their return they were forced to fight Omega Red.
While attending a conference in Ireland, Rev. Maddox decided to visit some older churches in the area and ran into Terry Cassidy. Terry was visiting her father's grave in the family church yard near Cassidy Keep. After some initial confusion about Maddox's identity, Terry and Maddox had a long discussion about God's plan. Maddox helped Terry in dealing with the death of her father and in the loss of her son.
At 'Cassidy Keep'; Declan McGuinness killed Finnan O'Bradaigh to seize the power of Craeliach and control every single spirit from the Faerie of Otherworld. Banshee returned home to stop with the help of the X-Men.
- Conall O'Reilly
- Declan McGuinness
- Donal O'Bradaigh
- Eamon O'Donnell
- Owen McGuinness
- Mrs. Malone