 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.He was re-elected 5 times, holding the seat until the 1826 general election, which he did not contest.