On 13 May 1919, he helped rescue his comrade Seán Hogan at gunpoint from a heavily guarded train at Knocklong station in County Limerick.

Breen, who was wounded, remembered how the battalion was "vehemently denounced as a cold-blooded assassins" and roundly condemned by the Catholic Church.

[9] After the fight, Seán Treacy, Séumas Robinson and Breen met Michael Collins in Dublin, where they were told to escape from the area.

Richard Mulcahy noted that British policy had "pushed rather turbulent spirits such as Breen and Treacy into the Dublin area".

Breen was present in December 1919 at the ambush in Ashtown beside Phoenix Park in Dublin where Martin Savage was killed while trying to assassinate the Lord Lieutenant of Ireland, Viscount French.

[14] Breen rejected the Anglo-Irish Treaty, which left him angry and embittered: "I would never have handled a gun or fired a shot … to obtain this Treaty … writing on the second anniversary of Martin Savage's death, do you suppose that he sacrificed his life in attempting to kill one British Governor-General to make room for another British Governor-General?

"[15]Although he had tried hard to avoid a conflict with his comrades after returning home from America in early 1922, Breen eventually joined the Anti-Treaty IRA in its fight against the Provisional Government of Ireland.

[21] Breen wrote a best-selling account of his guerrilla days, My Fight for Irish Freedom, in 1924, later republished by Rena Dardis and Anvil Press.

[2] In January 1927, he became the first anti-Treaty TD to take the Oath of Allegiance and sit in the Dáil Éireann after the establishment of the Irish Free State.

He returned to Ireland in 1932 following the death of his mother,[2] and regained his seat as a member of Fianna Fáil in the Dáil at that year's general election.

[25] Irish-American John S. Monagan visited Breen in 1948, and was surprised to see two pictures of Adolf Hitler, a medallion of Napoleon and a Telefunken radio.

"[28] Breen was co-chairman of the anti-Vietnam War organisation "Irish Voice on Vietnam" which he founded along with Peadar O'Donnell.

[33] Breen was, at the time, one of the most wanted men in Ireland, and South Tipperary was under martial law, yet a large celebration was held.

Many of the key members of the Third Tipperary Brigade attended, including flying column leaders Dinny Lacey and Hogan.

Breen was the brother in-law of Commandant Theobald Wolfe Tone FitzGerald, the painter of the Irish Republic Flag that flew over the GPO during the Easter Rising in 1916.

[36][page needed] Breen died in Kilcroney House, County Wicklow in 1969, aged 75, and was buried in Donohill, near his birthplace.

An estimated attendance of 10,000 mourners assembled in the tiny hamlet, giving ample testimony to the esteem in which he was held.