[1] He was killed aged 26, in Albert in France, on 5 July 1916, during the attack on Bouzincourt, a part of the Battle of the Somme.
His brother Dr J. Ivo O'Sullivan KM served with the 5th Connaught Rangers, as a medic in the war, in Ypres, Salonika, Gallipoli, earning a Military Cross.
[4] O'Sullivan ministered to the 7th Battalion, Royal Munster Fusiliers, and kept a diary from the day he left Killarney for the war until his death.
The soldier who had been ministered to by O'Sullivan when he was killed travelled to Killarney and met his mother Hannah.
But reburial of the war dead was disallowed by the British government at that time, with the sole exception of the Tomb of the Unknown Warrior.