Plunket was made a King's Counsel in 1795, and three years later was elected to the Irish House of Commons as a Member of Parliament for Charlemont. After the Act of Union was passed, Plunket lost his seat, and failed to be elected to Westminster for the University of Dublin in 1802, but he subsequently became Solicitor-General for Ireland in 1803, a post he held for two years before becoming Attorney-General for Ireland, again for two years. He was appointed a member of the Privy Council of Ireland on 6 December 1805. As Solicitor General he was one of the Irish officials singled out for attack in a series of scurrilous letters published under the name "Juverna". The author was soon discovered to be Robert Johnson, judge of the Court of Common Pleas (Ireland). Johnson was prosecuted for seditious libel, convicted and forced to resign from the Bench in disgrace.