William O'Brien, 2nd Marquess of Thomond KP PC (I) was an Irish peer and Chief of Clan O'Brien. He succeeded by special remainder as Marquess of Thomond in 1808 on the death of his uncle Murrough O'Brien, 1st Marquess of Thomond and was appointed a Privy Councillor and Knight of the Order of St Patrick on 11 November 1809. He was created Baron Tadcaster in the British Peerage in 1826. He was also Colonel of the disembodied Royal Cork City Militia in 1840.
- Occupation
- politician
- Position held
- Member of the Privy Council of Ireland, member of the House of Lords
- Born
- 1765
- Died
