Lemenagh Castle was both a castle (1480) and fortified house (1640) and was one of the principal strongholds of the O’Brien’s. The fortified house was built by Conor O’Brien who was married to Maura Rua or Red Mary. An inscription is found over the entrance porch in the following words: "This was built in the year of our Lord 1648, by Connor O’Brien, and by Mary-ni-Mahone, wife of the said Connor."

Often tower houses were completely incorporated into larger houses and buildings as thge need for security diminished. Here, an early seventeenth century addition was made by Maire Rua McMahon onto a late fifteenth century O'Brien tower house. Lemenagh Castle may be seen as the link in the transition from the early castles and tower houses of the fifteenth and sixteenth centuries and the later unfortified country houses of the landed gentry.

Conor O’Brien who was married to Maura Rua or Red Mary, returned from battle seriously injured, but she refused him admittance on the grounds that she believed he was dead. Eventually he was allowed in but died shortly thereafter, possibly by her hand. Red Mary remarried to improve her position.