After leaving school on 26 September 1907, just 11 days after his 14th birthday Seán MacMahon started work in Dublin Post Office.

He then turned to clerical work, possibly for Plunkett Bros. Maltsters who maintained a presence, office and malthouse perhaps, at 4 and 5 Sandwith Street.

Easter Week 1916 he fought as Captain of "B" Company, 3rd Battalion under Eamonn De Valera in Bolands Mills with responsibility for Westland Row Train Station.

He represented "B" Company at a meeting of the Battalion Council in 144 Great Brunswick Street on the Good Friday prior to the rising.

John Mc Mahon, he was still known by his birth name, and Capt Joseph O Connor cut telephone wires and seized signal boxes.

[2][3] Their first encounter was on Monday near Bath Avenue when MacMahon with seven of his men repelled soldiers from Beggars Bush Barracks and killed a sergeant major.

Captain MacMahon provided a solution by taking the man to St Vincent's Hospital himself while in uniform and under fire from the British troops.

De Valera, who was getting exhausted, did towards the end of the fighting order the burning of Westland Row station but had to be persuaded by MacMahon not to do so.

'When Lt James Fitzgerald assured his O/C he'd sit by him de Valera relented and fell asleep immediately.

The locals were friendly and sympathetic and they received an ovation in Grand Canal Street with offers of refuge.

May 1916 he was deported and imprisoned first in Wakefield then Frongoch where he was listed as John Mc Mahon 5B Sandwith Street and finally Wormwood Scrubbs.

"Seven of the most prominent members of the resident executive met at the headquarters of the printers union at 35 Lower Gardener Street to select a chief of staff in March 1918.

Those attending were Collins, Richard Mulcahy, Dick Mc Kee, Gearoid O Sullivan, Diarmuid O' Hegarty, Rory O’Connor and Seán MacMahon.

MacMahon 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.

John Kennedy was transferred from "B" Company, 3rd Battalion, Dublin Brigade, to his department to help run it.

Whenever there was an important meeting being held in the snug Richard Mulcahy would preside with MacMahon, Gearoid O Sullivan, Frank Thorton, Seán O’Connell, Michael Larken and of course Oscar Traynor these men were always present.

They were discussing whether to bring in a boat with arms"[14] He took part in several engagements during the Black and Tan war and was present during the night ambush when his Brother in Law, Leo Fitzgerald was killed in Great Brunswick Street on 14 March 1921.

Dublin had awoken on the morning to 14 March 1921, to the news that six IRA Volunteers, captured in an ambush at Drumcondra two months before, had been hanged.

Apparently the Auxiliaries had some inside information as they made straight for the local IRA headquarters at 144 Pearse Street.

The position became very difficult and, after many meetings with our Northern officers, it was decided that we would procure a quantity of arms under cover to be sent in to the six counties.

MacMahon was in Cork at this time and told Fitzgerald to hold things up because he was concerned that the guns may get into the wrong hands.

Things began to go badly wrong in London at the end of August 1922 and it seems that Fitzgerald may have been a rather incompetent gunrunner.

He messed up one revolver deal in June by not turning up for an appointment with the arms dealer who then sold the guns to Brazil.

The last word on the matter was in the Dáil in 1927 when Fianna Fáil's Frank Carney TD (Chief supplies officer Portobello 1922) commented that they fitted out a flying column from South Down/Armagh area two or three days before the attack on the Four Courts.

The leadership of the IRA organisation wished to continue the fight for an all island Republic using whatever means but particularly a well-armed nationalist army.

Mulcahy did secure a redundancy package for the men that would have to leave the National Army and return to civilian life.

The Army Council by now accepted and took steps to action the Governments budgetary target of 1,300 officers and 30,000 other ranks by January 1924.

The predicament the Army Council and in particular MacMahon and Mulcahy found themselves in was in large part due to their taking a stronger line against the IRA than would their political masters.

Here he stayed with his wife, Lucinda and 3 children Terence (Terry), Kathleen (Kanki) and Seán (Seánog).

Before the ceremony members of the Dublin Brigade IRA formed up outside 144 Pearse Street, the old Battalion HQ, under the command of Comdt Vincent Byrne and headed by a colour party under Mr P. Buttner and the St Brigid's Brass Band they marched to the Seán MacMahon Bridge.