The fourth castle, the present structure, was built by the MacNamara family after around 1425. Its builder may have been one Maccon Sioda MacNamara, chieftain of Clann Cuilein (i.e. the MacNamaras). He died before the castle was completed which happened under his son Sean Finn (died in 1467). At around 1500, Bunratty Castle came into the hands of the O'Briens (or O'Brians), the most powerful clan in Munster and later Earls of Thomond. They expanded the site and eventually made it their chief seat, moving it there from Ennis.