Most if not all of the British Soldiers around the GPO were from Irish Regiments including but not limited to Royal Irish Regiment, Royal Dublin Fusiliers, Royal Irish Fusiliers, Royal Inniskilling Fusiliers and Connaught Rangers.

The Royal Irish Regiment made up mostly of men from Dublin were the first to attack GPO and one RIR officer stated "they regarded, not unreasonably, everyone they saw as an enemy, and fired at anything that moved" which can be found in The Rising: Ireland, Easter 1916 page 148.

The Connaught Rangers patrolled around country side engaging Volunteers and liberating a town they took over. Some members of the Connaught Rangers who were on leave in Dublin at the time volunteered for temporary duty with the RIR and RDF the two regiments that did most of the fighting around the GPO.

The Royal Dublin Fusiliers even executed four men when they were guarding the Guinness brewery thinking they were members of Sinn Féin http://query.nytimes.com/mem/archive-free/pdf?res=940DEFDD173BE633A25757C1A9609C946796D6CF