Portarlington was a rotten borough and is a former United Kingdom Parliament constituency, in Ireland, returning one MP.
It was an original constituency represented in Parliament when the Union of Great Britain and Ireland took effect on 1 January 1801.
Dawson-Damer was appointed Comptroller of the Household, requiring a by-election.
Dunne was appointed Clerk of the Ordnance, requiring a by-election.
FitzPatrick succeeded to the peerage, becoming Lord Castletown, causing a by-election.