|Regiment/Unit:||Irish Guards, 2nd Battalion|
|Date of Death:||15-09-1916|
|Cause:||Killed in action, Battle of Flers-Courcelette (near Ginchy), Somme|
|Memorial:||Thiepval Memorial, Thiepval, Somme, France|
William was born in Co. Longford and enlisted in Dublin.
Pte O'Brien died during the Battle of the Somme, likely whilst serving with his battalion in the trenches, north-east of Ginchy, in an attack on the German line near Flers and trenches near Les Boeufs, as part of the Battle of Flers-Courchette. This began early on the morning of the 15 September, and is remembered as the day tank warfare was introduced.
O'Brien was one of 300 casualties from other ranks in his battalion from this day alone. His sister, Mrs. Margaret Kavanagh, was his next-of-kin.