In October 1917, during his second year at Belvedere, aged 15, he joined the IRA.
Assigned originally to ‘C’ Company 1st Battalion, based on the north side of Dublin, he later transferred to the newly formed ‘H’ Company, under the command of Capt.
Seamus Kavanagh.
His first job as a member of the IRA was delivering mobilisation orders around the city.
Along with other volunteers, Barry trained in a number of locations in Dublin, including the building at 44 Parnell Square, the present day headquarters of Sinn Féin, now named Kevin Barry Hall.
The IRA held Field exercises during this period which were conducted in north county Dublin and in areas such as Finglas.

The following year, at the age of 16, he was introduced by Seán O'Neill and Bob O'Flanagan to the Clarke Luby Club of the IRB, which had been reorganised.
He took part in a number of IRA operations in the years leading up to his capture.
He was part of the unit which raided the Shamrock Works for weapons destined to be handed over to the R.I.C. He also took part in the raid on Mark's of Capel Street, looking for ammunition and explosives.
On 1 June 1920, under Vice-Commandant Peadar Clancy, he played a notable part in the seizing of the King's Inn, capturing the garrison’s arms.
The haul included 25 rifles, two light machine guns and large quantities of ammunition.
The 25 British soldiers captured during the attack were released as the volunteers withdrew.
In recognition of his dedication to duty he was promoted to Section Commander.
In October 1917, during his second year at Belvedere, aged 15, he joined the IRA. Assigned originally to ‘C’ Company 1st Battalion, based on the north side of Dublin, he later transferred to the newly formed ‘H’ Company, under the command of Capt. Seamus Kavanagh. His first job as a member of the IRA was delivering mobilisation orders around the city. Along with other volunteers, Barry trained in a number of locations in Dublin, including the building at 44 Parnell Square, the present day headquarters of Sinn Féin, now named Kevin Barry Hall. The IRA held Field exercises during this period which were conducted in north county Dublin and in areas such as Finglas. 
The following year, at the age of 16, he was introduced by Seán O'Neill and Bob O'Flanagan to the Clarke Luby Club of the IRB, which had been reorganised. He took part in a number of IRA operations in the years leading up to his capture. He was part of the unit which raided the Shamrock Works for weapons destined to be handed over to the R.I.C. He also took part in the raid on Mark's of Capel Street, looking for ammunition and explosives. On 1 June 1920, under Vice-Commandant Peadar Clancy, he played a notable part in the seizing of the King's Inn, capturing the garrison’s arms. The haul included 25 rifles, two light machine guns and large quantities of ammunition. The 25 British soldiers captured during the attack were released as the volunteers withdrew. In recognition of his dedication to duty he was promoted to Section Commander.
Kevin Barry