The powerful MacNamara family built the present structure around 1425 but by 1475 it had became the stronghold of the O'Briens, the largest clan in North Munster.
The chief of the Clan Cullein (which was led by the MacNamara family) was Maccon MacNamara. His father had previously conquered Quin Castle from the Normans, and it having been wrecked in the process he had donated it to the church to be converted into a Franciscan monastery. Maccon had kept up the family tradition of piety, and even received a personal thanks in an encyclical from Pope Eugenius IV. Perhaps inspired by the Normans, the MacNamara also built around 50 towers and castles around Thomond during this time, and Bunratty was one such castle. In 1475, the castle became the property of the O’Briens – almost certainly not through conquest, as the relationship between the MacNamara and O’Brien clans was very close, but possibly through marriage or in tribute. They expanded the castle and at some point before 1558, it became the capital of Thomond.