Donough O'Brien, 2nd Earl of Thomond

Donough O'Brien, 2nd Earl of Thomond (Irish: Donnchadh Ó Briain; died 1 April 1553), also known as "the fat", was the son of Connor O'Brien, King of Thomond and Annabell Burke. He inherited the earldom from his uncle, Murrough O'Brien, by special remainder.

O'Brien married Helen Butler, daughter of Piers Butler, 8th Earl of Ormonde and Lady Margaret Fitzgerald. He died on 1 April 1553, after being attacked by his brothers at the family seat of Clonroad. O'Brien's brother Sir Donald was named king of Thomond by the Dalgais, but O'Brien's son, Connor, allied himself with the English and regained control of his lands.

Children of Donough O'Brien and Helen Butler:

• Margaret O'Brien (d. 1568) married Dermod O'Brien, 2nd Baron Inchiquin, and Richard Burke, 2nd Earl of Clanricarde
• Connor O'Brien, 3rd Earl of Thomond (c. 1534 – 1581)
• Donal or Daniel
• Honora married Teige Macnamara