In a memorandum sent by General Sir John Maxwell to the then British Prime Minister, Herbet Asquith, the following description was provided for Sean Heuston:

This man was in command of the Mendicity Institute, Usher's Island. One British Officer and nine men were killed by the fire from the building which had to be carried by assault. Twenty-three rebels were captured in it amongst them this man, and large stores of revolver and rifle ammunitions and bombs were found. Orders and despatches were also discovered showing that this man was in constant communication with the leaders. In all of these despatches he described himself and was described as Captain.

Sean Heuston, W. O'Dea, P. Kelly and J. Crenigan were tried together by Field General Courts Martial on 4 May 1916. The proceedings are contained in the PRO document WO 71/351.

Court Martial Proceedings

The members of the courts martial were Colonel E.W.S.K. Maconchy (President), Lieutenant-Colonel Bent and Major F.W. Woodward.

To the charge of " ... did take part in an armed rebellion and in the waging of war against His Majesty the King, such act being of such a nature as to be calculated to be prejudicial to the Defence of the Realm and being done with the intension and for the purpose of assisting the enemy."

All four defendants pleaded not guilty.

The 1st witness was Captain A.W. MacDermot (7th Royal Dublin Fusiliers) who stated

On 26 April I was present when the Medicity Institution was taken by assault by a party of the 10th Royal Dublin Fusiliers. Twenty-three men surrendered on that occasion. I identify the four prisoners as having been in the body of men who surrendered. They left their arms except their revolvers in the Mendicity Institute when they surrendered. Some of them still wore revolvers. One officer of the 10th Royal Dublin Fusiliers was killed and 9 men wounded by fire from this Institute on the 24th April. I searched the building when they surrendered. I found several rifles, several thousand rounds of ammunition for both revolvers and rifles. I found 6 or 7 bombs charged and with fuses in them ready for use.

I found the following papers: An order signed by James Connolly, one of the signatories to the Irish Republic Proclamation, directing "Capt. Houston" (Sic) to "Seize the Mendicity at all costs." Also papers detailing men for various duties in the Mendicity Institute. All these papers are headed "Army of the Irish Republic." Also two message books signed by Heuston "Capt." One contains copies of messages sent to "Comdt. General Connolly" giving particulars of the situation in the Institute. The other message book contains copies of messages commencing on the 22nd April two days before the outbreak. One message contains a reference to MacDonagh who is stated to have just left Heuston. Another is a message to "all members of D Coy. 1st Batn." stating that the parade for the 23rd is cancelled and all rumours are to be ignore. Another message dated the 23rd states "I hope we will be able to do better next time."

Capt. MacDermot then testified that Heuston commanded the party of men who surrendered.

The 2nd witness was Lieutenant W.P. Connolly (10th Royal Dublin Fusiliers) stated

I was present when 23 men surrendered on the 26th April at the Mendicity Institute. I identify the four prisoners before the court as being amongst them. The leader was J.J. Heuston. I was present when the troops were fired on from the Mendicity Institute on the 24th April, when Lieutenant G.A. Neilan* was killed and 6 men wounded to my knowledge. Heuston was without a coat when he surrendered and also had no hat on. He was not in the uniform of the Irish Volunteers. I was present when the building was searched and found arms and ammunition in it and also the documents now before the court. Among the arms there were some old German Mausers. Among the ammunition there were two cardboard boxes of "Spange" German ammunition.

[*Lieutenant G.A. Neilan (10th Royal Dublin Fusiliers) was subsequently buried in Glasnevin Cemetery.]

When cross-examined by Sean Heuston, Lieutenant Connolly was not able to say exactly where, in the building, he had found the message books.

In his defence W.O'Dea stated

I was perfectly ignorant of what was going to occur. I understood it was an ordinary route march when I was called out as we had been told for some time previously that the best equipped Company was to get a prize at the Easter Manoeuvres. It was to have taken place on Easter Sunday but was postponed. I do not know why it was postponed. I turned out in full uniform but I took it off when we were about to surrender.

In his defence Sean Heuston stated

The message in the notebook produced saying "I hope we will be able to do better next time" is not mine.

In his defence P. Kelly stated

I did not know anything about the rebellion beforehand or what I was coming out for. I came out because I was asked to. I thought it was for manoeuvres. I did not fire any shots.

In his defence J. Crenigan stated

I did not know what I was called out for. I though it was for manoeuvres. I am 16 years old.

Court Martial Verdict

The court found all four defendants guilty. Heuston, O'Dea and Kelly were sentenced to death by shooting. Crenigan was sentenced to two years' imprisonment, with a recommendation for mercy on account of his age.

General Maxwell commuted the death sentences on O'Dea and Kelly to three years' imprisonment. The death sentence passed on Heuston was confirmed by General Maxwell.

Between 3.45 and 4.05am on 8 May 1916, Sean Heuston was shot in the former stonebreakers yard at Kilmainham Prison. His remains were later buried in Arbour Lane Cemetery.