In the 12th century , the O'Briens , who were Kings of Thomond , left their seat of power in Limerick .
In the 12th century , the O'Briens , who were Kings of Thomond , built a royal residence at Clonroad on the then island .
In the 12th century , the O'Briens , who were Kings of Thomond , left their seat of power in Limerick and built a royal residence at Clonroad on the then island .