This is a list of by-elections to Dáil Éireann, the house of representatives of the Oireachtas, the Irish legislature. By-elections in Ireland occur to fill vacant seats which can be caused by the death, resignation, disqualification or expulsion of a sitting Teachta Dála. Under the Electoral Act 2011, the writ for the by-election must be issued within six months of the vacancy. There have been 131 by-elections since 1923, to fill 133 vacancies. 92 of these were caused by the death of a sitting Teachta Dála. There were seven by-elections during the lifetime of the 31st Dáil. There were no by-elections during the 3rd, 7th, 9th, 11th, 22nd, 25th and 26th Dála. The longest period without a by-election was almost 10 years between 1984 and 1994. The largest number of by-elections on one day was on 11 March 1925, when seven constituencies filled nine vacancies caused by the National Party's split from Cumann na nGaedheal. Those seven by-elections included two which filled two vacancies, via the single transferable vote. All the other by-elections have used its single-winner analogue, the alternative vote. Twenty-three TDs who were elected at a by-election were not subsequently re-elected at a general election. The only person twice elected at by-elections was Thomas Hennessy.
By-elections in which seats changed hands are indicated with a grey background.
Vacancies not filled
When Pierce McCan died on 6 March 1919, his East Tipperary seat was left vacant at Westminster. In April 1919 a Dáil committee considering how to fill the vacancy considered allowing nomination by the Labour Party before recommending that the Sinn Féin constituency organisation should nominate. However, in June 1919 the Dáil decided that "it was due to the memory of the late Pierce McCann that his place should not be filled at present". Later vacancies were also left unfilled; when Diarmuid Lynch resigned his seat in 1920, Arthur Griffith said "as the letter of resignation was addressed to the people of South-East Cork, the next step in the matter lay with the South-East Cork Executive of Sinn Fein". In the first Dáil, four Sinn Féin TDs represented two separate constituencies: Éamon de Valera, Arthur Griffith, Eoin MacNeill and Liam Mellowes. Ordinarily, this would prompt them to choose one constituency to represent, and to move a writ for a by-election in the other constituency.