Doonbeg Castle (Gaelic: Dun Beag = the small fort) is an Irish tower castle from the 16th century. It was originally built for Daniel O'Brien, the Earl of Thomond. Turlough MacMahon of West Clare took Doonbeg in 1585. Some 10 years later, 1595, O'Brien reclaimed Doonbeg. It overlooks the Doonbeg River which flows into the bay of Dunmore on the Atlantic coast. The castle is about seventy feet high.
Two families of Country Clare are related to this tower house - the clan MacMahon and the clan O'Brien. The genealogy of clan MacMahon and the O'Brein family shows that they have a common Irish ancestor - Mahon, son of Murtagh Mor who was a grandson of Brian Boru. Brian Boru, High King of Ireland, descended from Cennedig of the royal clans of Munster.