[1] He was educated at Mount Sion Christian Brothers School and later in Thurles, County Tipperary, where his father was the postmaster.

He was second-in-command to Thomas Ashe (who later died on hunger strike) in an encounter with the armed Royal Irish Constabulary (RIC) at Ashbourne, County Meath during the Easter Rising in 1916—one of the few stand-out victories won by republicans in that week, and generally credited to Mulcahy's grasp of tactics.

On his release, Mulcahy immediately rejoined the republican movement and became commandant of the Dublin Brigade of the Irish Volunteers.

Mulcahy served as Minister for Defence in the new Free State government from January 1924 until March 1924,[7] but resigned in protest because of the sacking of the Army Council after criticism by the Executive Council over the handling of the 'Army Mutiny', when some National Army War of Independence officers almost revolted after Mulcahy demobilised many of them at the end of the Civil War.

However, he secured election to Seanad Éireann as a Senator, the upper house of the Oireachtas, representing the Administrative Panel.

Facing his first general election as party leader, Mulcahy drew up a list of 13 young candidates to contest seats for Fine Gael.

Following the 1948 general election—at which, following boundary changes, Mulcahy was elected for Tipperary South, the dominant Fianna Fáil party finished six seats short of a majority.

However, it was 37 seats ahead of Fine Gael, and conventional wisdom suggested that Fianna Fáil was the only party that could form a government.

He played a leading role in persuading the other parties to put aside their differences and join forces to consign the then Taoiseach and Fianna Fáil leader Éamon de Valera, to the opposition benches.

Many Irish republicans had never forgiven Mulcahy for his role in the Civil War executions carried out under the Cosgrave government in the 1920s.

Mulcahy stepped aside and encouraged his party colleague John A. Costello, a former Attorney General, to become the parliamentary leader of Fine Gael and the coalition's candidate for Taoiseach.

Richard Mulcahy married Min Ryan, the former fiancée of Seán Mac Diarmada, in 1920, and lived in a flat in Oakley House, Ranelagh.

[9] Min, a member of the Cumann na mBan Executive, had herself been involved in nationalist activity at the time of the Rising.