British forces initially put their efforts into securing the approaches to Dublin Castle and isolating the rebel headquarters, which they believed was in Liberty Hall. The British commander, Brigadier-General W. H. M. Lowe, worked slowly, unsure of the size of the force he was up against, and with only 1,269 troops in the city when he arrived from the Curragh Camp in the early hours of Tuesday 25 April. City Hall was taken on Tuesday morning. The rebel position at St Stephen's Green, held by the Citizen Army under Michael Mallin, was made untenable after the British placed snipers and machine guns in the Shelbourne Hotel and surrounding buildings. As a result, Mallin's men retreated to the Royal College of Surgeons building. British firepower was provided by field artillery summoned from their garrison at Athlone which they positioned on the northside of the city at Phibsborough and at Trinity College, and by the patrol vessel Helga, which sailed upriver from Kingstown. Lord Wimborne, the Lord Lieutenant, declared martial law on Tuesday evening.
At Ashbourne, County Meath, the North County Dublin Volunteers (also known as the Fingal Volunteers), led by Thomas Ashe and his second in command Richard Mulcahy, attacked the RIC barracks. Reinforcements came from Slane and after a five-hour battle, the Volunteers captured over 90 prisoners. There were 8–10 RIC deaths and two Volunteer fatalities, John Crennigan and Thomas Rafferty. The action pre-figured the guerrilla tactics of the Irish Republican Army in the Irish War of Independence from 1919 to 1921. Elsewhere in the east, Seán MacEntee and County Louth Volunteers killed a policeman and a prison guard. In County Wexford, the Volunteers took over Enniscorthy from Tuesday until Friday, before symbolically surrendering to the British Army at Vinegar Hill – site of a famous battle during the Irish Rebellion of 1798.