Under the influence of his father, Austin Stack joined the local Young Ireland Society and the local branch of the Irish National Foresters, and was a keen student of Irish history.

Appalled at the Cinderella status of Gaelic football and hurling, he co-founded the John Mitchel club in Tralee and became its secretary. The remarkable success of this club led to the general reorganisation of the Gaelic Athletic Association in the county. Besides his flair for organisation Stack was a regular member of the county football team from 1902 until he had to retire from the game, due to injury, in 1908. He served as secretary of the Kerry GAA county board (1904–8) and as chairman (1914–17). Later he was a member of the Munster and central councils. He was among those who urged the revival of the Tailteann games (Aonach Tailteann), which took place in 1924, 1928, and 1932.

When visiting Tralee on 27 February 1916, Pádraig Pearse informed Stack that plans had been prepared by the IRB’s military council for an insurrection, that arms were to be landed at Fenit pier on Easter Sunday/Monday, that the Tralee Volunteers were to be responsible for this operation, and that the landed arms were to be used firstly to arm the Kerry Volunteers and then to be distributed to the Clare, Cork, Galway, and Limerick Volunteers. Subsequently Stack secretly and meticulously prepared for the operation. However, unaware that the Aud was arriving three days earlier than the scheduled date (23 April), he failed to make contact with it. His carefully prepared arrangements were further upset by the unexpected arrival of Sir Roger Casement on Banna Strand on Good Friday morning and his subsequent arrest. That evening Stack, on the pretext of wishing to consult a comrade detained by the RIC, walked into the police barracks and was arrested.

Stack was originally sentenced to death for his activities during 1916. As a Sinn Féin abstentionist candidate, he was elected MP for Kerry West in 1918. He fought in the Irish Civil War against the Treaty, was captured by government troops and spent 41 days on hunger strike. He was released in 1924.

In December 1925 Stack unsuccessfully proposed that De Valera lead his abstentionist colleagues into Dáil Éireann to defeat the settlement negotiated under the auspices of the boundary commission. When De Valera and his supporters split from Sinn Féin (March 1926), Stack, as joint secretary, continued to be a major figure in the party. The establishment of Fianna Fáil seriously damaged the party, which could put forward only fifteen candidates in the general election of June 1927. After the eleventh count, Stack retained his seat in Kerry and Limerick West. In another general election (September 1927) Sinn Féin, owing to lack of funds, was not able to put forward candidates, and thereby Stack’s political career ended. He continued, however, to be involved with Sinn Féin and the ‘republican government’. From 1926 onwards he had begun to attend law lectures in Dublin. He became ill with appendicitis and died 27 April 1929 at the Mater hospital, aged 49, Dublin; he was buried in Glasnevin cemetery. A portrait by Leo Whelan is in the Municipal Gallery of Modern Art, Dublin.

On 10 August 1925 he married Winifred Una Gordon, the widow of an RIC inspector; he had stayed in her house while on the run in 1920.

Austin Stack Park in his home town of Tralee, one of the Gaelic Athletic Association’s stadiums, is named in his honour, as is the Austin Stacks Hurling and Gaelic football club.