On 31 October, Royal Irish Constabulary (RIC) district-inspector Philip Kelleher was shot dead in Granard, and the Auxiliary Division proceeded to set fire to parts of the town.

In a separate attack on 8 November, Mac Eoin led his men against the RIC at Ballinalee which killed an eighteen-year-old constable and wounded three others.

[7] The small RIC garrison allegedly sang "God Save the King" as they took up positions to return fire.

[citation needed] On the afternoon of 7 January 1921, a joint RIC-British Army patrol consisting of eleven policemen and nine soldiers arrived on the street outside the home of Anne Martin in Kilshruley, where MacEoin was staying.

"[8] The RIC officer Mac Eoin killed was district-inspector Thomas McGrath; he also wounded a police constable as well.

At the Clonfin Ambush, Mac Eoin ordered his men to care for wounded prisoners of war at the expense of captured weaponry.

[13] Mac Eoin was captured at Mullingar railway station in March 1921, imprisoned and sentenced to death for the murder of McGrath.

In June 1921, Wilson was petitioned for clemency by Mac Eoin's mother (who referred to her son as "John" in her letter), by his own brother Jemmy and by the local Church of Ireland vicar, and passed on the appeals out of respect for the latter two individuals.

[citation needed] Mac Eoin wrote the following letter to his friend (and classmate at Moyne Latin School) Father Jim Sheridan, a combatant in the Old IRA and a 'flying column' member, who had been ordained and sent to Milwaukee to study theology: Dear Jim, Last week I was tried, convicted and sentenced to die three weeks from today.

Six IRA members, led by Paddy Daly and Emmet Dalton, captured an armoured car and, wearing British Army uniforms, gained access to Mountjoy Prison.

[16] He was eventually released from prison — along with all other members of the Dáil, after Collins threatened to break off treaty negotiations with the British government unless he were freed.

During the Civil War he pacified the west of Ireland for the new Free State, marching overland to Castlebar and linking up with a seaborne expedition that landed at Westport, County Mayo.

He was one of a number of Free State generals whom IRA deputy commander Ernie O'Malley ordered to be shot on sight for ill-treating prisoners.

[17] Mac Eoin was later appointed to head the Curragh training camp in August 1925, quartermaster general in March 1927 and chief of staff in February 1929.

He unsuccessfully stood twice as candidate for the office of President of Ireland, against Seán T. O'Kelly in 1945 and Éamon de Valera in 1959.