On Monday, 24 April 1916, 220 members of the ICA (including 28 women) took part in the Easter Rising, alongside a much larger body of the Irish Volunteers. They helped occupy the General Post Office (GPO) on O'Connell Street (then named Sackville Street), Dublin's main thoroughfare. Michael Mallin, Connolly's second-in-command, along with Kit Poole, Constance Markievicz and an ICA company, occupied St Stephen's Green. Another company under Sean Connolly took over City Hall and attacked Dublin Castle. Finally, a detachment occupied Harcourt Street railway station. ICA men were the first rebel casualties of Easter week, two of them being killed in an abortive attack on Dublin Castle. The confusion in the chain of command caused conflict with the Volunteers. Harry Colley and Harry Boland came out from their outposts in the Wicklow Chemical Manure Company's office 200 yards away, where they were under the command of an irascible officer, Vincent Poole; the post had been set up by James Connolly, without countermanding orders from affective Volunteers.