O'Sullivan, then 25, was the youngest IRB officer fighting in the GPO. He had been personally chosen by rebellion leader Seán Mac Diarmada to serve as his aide-de-camp. The additional honour of raising the Irish flag was given to him by another of the rebellion's leaders, Patrick Pearse. He married Maud Kiernan, sister of Kitty Kiernan, daughter of Peter and Bridget Kiernan of the Greville Arms Hotel, Granard, County Longford, with whom he had three daughters and a son. His wife died at age 40 and he remarried, to Mary ("Mae") Brennan of Belfast.
