The War Office ordered that Kevin Barry be tried by court-martial under the Restoration of Order in Ireland Act, which received Royal Assent on 9 August 1920.
General Sir Nevil Macready, Commander-in-Chief of British forces in Ireland then nominated a court of nine officers under a Brigadier-General Onslow.

On 20 October, at 10 o’clock, the nine officers of the court — ranging in rank from Brigadier to Lieutenant — took their places at an elevated table.
At 10.25, Kevin Barry was brought into the room by a military escort.
Then Seán Ó hUadhaigh sought a short adjournment to consult his client.
The court granted this request.
After the short adjournment Barry announced "As a soldier of the Irish Republic, I refuse to recognise the court."
Brigadier Onslow explained the prisoner's "perilous situation" and that he was being tried on a capital charge.
He did not reply.
Ó hUadhaigh then rose to tell the court that since his client did not recognise the authority of the court he himself could take no further part in the proceedings.

Barry was charged on three counts of the murder of Private Marshall Whitehead.
One of the bullets taken from Whitehead's body was of .45 calibre, while all witnesses stated that Barry was armed with a .38 Mauser Parabellum.
The Judge Advocate General informed the court that the Crown had only to prove that the accused was one of the party that killed three British soldiers, and every member of the party was technically guilty of murder.
In accordance with military procedure the verdict was not announced in court.
He was returned to Mountjoy, and at about 8 o’clock that night, the district court-martial officer entered his cell and read out the sentence: death by hanging.
The public learned on 28 October that the date of execution had been fixed for 1 November.
The War Office ordered that Kevin Barry be tried by court-martial under the Restoration of Order in Ireland Act, which received Royal Assent on 9 August 1920. General Sir Nevil Macready, Commander-in-Chief of British forces in Ireland then nominated a court of nine officers under a Brigadier-General Onslow.
On 20 October, at 10 o’clock, the nine officers of the court — ranging in rank from Brigadier to Lieutenant — took their places at an elevated table. At 10.25, Kevin Barry was brought into the room by a military escort. Then Seán Ó hUadhaigh sought a short adjournment to consult his client. The court granted this request. After the short adjournment Barry announced "As a soldier of the Irish Republic, I refuse to recognise the court." Brigadier Onslow explained the prisoner's "perilous situation" and that he was being tried on a capital charge. He did not reply. Ó hUadhaigh then rose to tell the court that since his client did not recognise the authority of the court he himself could take no further part in the proceedings.
Barry was charged on three counts of the murder of Private Marshall Whitehead. One of the bullets taken from Whitehead's body was of .45 calibre, while all witnesses stated that Barry was armed with a .38 Mauser Parabellum. The Judge Advocate General informed the court that the Crown had only to prove that the accused was one of the party that killed three British soldiers, and every member of the party was technically guilty of murder. In accordance with military procedure the verdict was not announced in court. He was returned to Mountjoy, and at about 8 o’clock that night, the district court-martial officer entered his cell and read out the sentence: death by hanging. The public learned on 28 October that the date of execution had been fixed for 1 November.
Kevin Barry