Caisleán na nGrágán aka Gragan Castle - the original 15th/16th century tower house - was the seat of the chief of the "Prince of Burren" - Ó Lochlainn alias O'Loughlen.
This "Castell of the Gragans" lent it's name to "the Barony of Gragans" up until the end of the 16th century.
In 1641 Gregans Castle was the property of Owney Mór O'Loughlen was proprietor of Gregans in 1641. These O'Loughlens were dispossessed soon after in 1854 by Cromwell's "to hell or to Connaught" settlement.
The land was then granted to George Óg Martyn (the son of Turlough O'Loughlen & Alice Martyn, daughter of George Martyn (Mayor of Galway 1632-1633).
Rather than extending the existing tower house (as was often the case) the Martyns built another fortified structure nearby.
Gregans Castle Hotel (across the road) is a newer Georgian manor house.