On Easter Monday Captain Seán Connolly of the Irish Citizen Army and his company of approximately thirty men convened at Liberty Hall. They were directed to seize and hold City Hall adjoining Dublin Castle. They marched up College Green and Dame Street and occupied City Hall and adjacent buildings, including the Evening Mail premises on the corner of Parliament Street and Dame Street. The ICA contingent in City Hall included Dr Kathleen Lynn and Helena Molony.

A party of six men approached the gate of Dublin Castle and shot dead the unarmed policeman on duty, Constable James O’Brien of the Dublin Metropolitan Police, as he tried to close the gate to prevent entry. The party then seized the guardroom, disarming and imprisoning the six soldiers present. In the plans for the Rising, it appears that Dublin Castle was not selected as one of the positions to be commandeered, presumably because it was believed that it was too well guarded. On the day, however, apart from sixty-five wounded service men in the hospital, there were fewer than twenty-five soldiers on duty. Moreover, at the very time the ICA men captured the guardroom, the under-secretary, Sir Matthew Nathan and two of his officials were in conference in the Castle; unaware that the Rising had actually broken out they were arranging for the arrest of those they suspected of being involved in planning an insurrection. It would certainly have been a tremendous coup for the Provisional Government if Dublin Castle, the symbol of British dominion, and one of the most senior officials of the British administration were captured.

The City Hall and the other occupied buildings soon came under intense rifle and machine gun fire. In the course of the afternoon Connolly was shot and killed by a sniper on the Bedford Tower in Dublin Castle. City Hall and the associated positions were assaulted by superior numbers later that evening. Following a number of casualties the forces surrendered that night.