Cyril Falls joined the 11th battalion of the Royal Inniskilling Fusiliers, part of the 36th (Ulster) division, which had in its origins been the paramilitary Ulster Volunteer Force that Sir Edward Carson had raised in order to stop the British government introducing Home Rule to Ireland. Falls married in 1915, and that year went to the Western Front, where he stayed for the rest of the war. He was on the staff of the 36th division and then the 62nd division, and then became a liaison officer with the French, with the rank of captain. He had a distinguished career: as Michael Howard notes, “he was twice mentioned in dispatches and twice cited for the Croix de Guerre.”
So far my sole information on Falls is the piece on him by Michael Howard in the Oxford Dictionary of National Biography.