[1] Portarlington was founded in 1666, by Sir Henry Bennet, who had been Southern Secretary to Charles II and to whom that King, on his restoration, had made a grant of the extensive estates of Ó Díomasaigh, Viscount Clanmalier, confiscated after the Irish Rebellion of 1641.

In accordance with this enactment the borough created in Cooletoodera (Cúil an tSúdaire), received the name of Port-Arlington, or Arlington's Fort.

The rebellion of 1798 resulted in several local men from Lea castle being apprehended and subsequently put to death by hanging in the town's market square.

This extract shows that a corporation of 15 people was responsible for the re-election of nominees of the Dawson family to parliament, many of whom were non-resident, to represent the town of 2,800 people: Prior to the legislative Union between Great Britain and Ireland, this borough sent two Members to the Irish Parliament; since 1800 it has returned one to the Imperial Parliament, and so close has been this corporation, that for 50 years previous to the last general election, the nominee of the Dawson family, commonly a total stranger to the borough, was always returned without a contest.

[citation needed] Crossing the river into County Offaly, the land becomes marshy and wet with extensive peat bogs.

[1] According to the 2022 census, 79.2% of inhabitants were born in Ireland, with Poland (5%), the UK (4.4%), and elsewhere in the EU (4.8%) representing other places of birth of the population.

[1] Reflecting the development of new housing in the early 21st century, the same census suggested that a majority of residents (59.81%) were living in private homes built after 2001.

[1] Portarlington is a focal point of the Irish railway network, being situated on the junction for services to the west (Galway and County Mayo), the south (Cork, Limerick, Tralee) and the east (Dublin, Kildare).

[21] Public transport by road includes, as of January 2017, one intercity bus service which operates direct to Dublin.

[citation needed] There is a local-link town service operated by Slieve Bloom Coaches linking Portarlington with Portlaoise and also with Tullamore.

[citation needed] A weekday service by JJ Kavanagh and Sons to NUI Maynooth University and Institute of Technology, Carlow operates during the college term.

Other developments included the addition of two all-weather soccer pitches,[citation needed] and improvements to the People's Park.

[citation needed] The People's Museum, situated within the Catholic Club on Main Street in Portarlington is small but holds many different exhibits ranging from local memorabilia to a Bronze Age Celtic dagger.

Portarlington, its Savoy cinema (now closed) and the nearby Lea Castle appeared in the 1993 Irish film Into the West.

[25] The festival has typically run over a weekend, with live music, dance, sport, history, food and a parade.