Portarlington was a rotten borough and was a United Kingdom Parliament constituency in Ireland, returning one MP from 1801 to 1885. It was an original constituency represented in Parliament when the Union of Great Britain and Ireland took effect on 1 January 1801.
This constituency was the parliamentary borough of Portarlington in Queen's County (renamed County Laois in 1920).
A report into the boundaries was undertaken in 1831. The Parliamentary Boundaries (Ireland) Act 1832 defined the boundaries of the parliamentary borough as:
Notable MPs included Hon. William Lamb, who, as Viscount Melbourne, served as prime minister briefly in 1934 and again from 1835 to 1841.
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.