 He was the son of Sir Lucius O'Brien, 3rd Baronet (1731–1795) and Anne French.On his father's death (possibly as the result of a duel), he inherited Sir Lucius' baronetcy and his seat in the Parliament of Ireland, representing Ennis from 1795 until the Union with Great Britain in 1801.At the 1802 general election, O'Brien was elected as one of the two Members of Parliament (MPs) for Clare.