It replaced the near-identical office of President of the Executive Council, which had been the Irish premier's title since the creation of the Irish Free State in 1922. The first President of the Executive Council, W.T. Cosgrave, is counted as the first Taoiseach, even though he never officially held that exact title.
Below is a timeline beginning with the establishment of the First Dáil in 1919 and ending with the present day, showing the Oireachtas careers of the thirteen men to have been Taoiseach to date.
- While W.T. Cosgrave is regarded as the first President of the Executive Council/Taoiseach, the unrelated titles President of the Dáil and President of the Irish Republic existed in the years between the establishment of Dáil Éireann in 1919 and the creation of the Irish Free State in 1922. These titles were held by Cathal Brugha (Jan-Apr 1919), Éamon de Valera (1919-1922), and Arthur Griffith (Jan-Aug 1922). Michael Collins held the post of Chairman of the Provisional Government of Southern Ireland, for the same time as Griffith was President of the Republic. None of these titles are shown on the timeline, except for de Valera's time as President of the Republic, as he became Taoiseach later.
- All thirteen Taoisigh were alive at the same time from Brian Cowen's birth in January 1960 until W.T. Cosgrave's death in November 1965.
- The only Taoiseach who had previously served as a Senator was Garret FitzGerald, who sat in the Seanad from 1965 until 1969.
- The only Taoiseach to also become President of Ireland was Éamon de Valera, who served as President from 1959 until 1973. Albert Reynolds ran to become the Fianna Fáil presidential candidate in 1997, but lost the nomination to Mary McAleese.
- Seán Lemass, Albert Reynolds and Brian Cowen are the only Taoisigh who never served as Leaders of the Opposition
- The longest-lived Taoiseach is Liam Cosgrave, at 96.
- Éamon de Valera holds the record for most cumulative time as Taoiseach (21 years and 2 months).
- John Bruton holds the record for least cumulative time as Taoiseach (2 years, 6 months).
- The youngest man to enter the office was W.T. Cosgrave, who was 42 years old when he became President of the Executive Council in 1922. Éamon de Valera was the oldest Taoiseach during his last term from 1957 to 1959. He was 78 years old when he stepped down.
- The oldest man to enter the office for the first time was Séan Lemass, who was 59 years old when he became Taoiseach in 1959. Enda Kenny was just a month younger than him when he became Taoiseach in 2011.
- The 21st Dáil (1977-1981) contained the most Taoisigh (past, present and then-future) in the Dáil at once, with 8: Lynch, Liam Cosgrave, Haughey, FitzGerald, Reynolds, Bruton, Ahern and Kenny.