Thomas McMahon was born in Cookstown. He enlisted in Belfast. Rifleman Thomas McMahon was serving with the 2nd Battalion of the Royal Irish Rifles when he was killed in action on Friday 6th September 1918 close to Ploegsteert Wood. Rifleman McMahon was killed during an attack that was aimed at dislodging the enemy from trenches which they had recently taken in the area around Armentieres and Ploegsteert Wood. The battalion lost thirty-three in the action.
The Ploegsteert Memorial stands in Berks Cemetery Extension, which is located 12.5 Kms south of Ieper town centre, on the N365 leading from Ieper to Mesen (Messines), Ploegsteert and on to Armentieres. From Ieper town centre the Rijselsestraat runs from the market square, through the Lille Gate (Rijselpoort) and directly over the crossroads with the Ieper ring road. The road name then changes to the Rijselseweg (N336). 3.5 Kms along the N336 lies a fork junction with the N365. The N365, which forms the right hand fork, leads to the town of Mesen. The Cemetery lies 3 Kms beyond Mesen on the right hand side of the N365, and opposite Hyde Park Corner Royal Berks Cemetery.