On 30 January 1972, McKay was part of a patrol on the streets of Derry involved in the killing of unarmed civilians known as Bloody Sunday.

In official reports concerning the massacre he is referred to as Soldier T. An inquiry concluded that McKay was likely responsible for firing the shot which wounded Patrick Brolly.

The inquiry found that if he did fire the shots which wounded Brolly, he unintentionally hit him while aiming at another man who was throwing bottles containing acid at him.

As 4 Platoon's advance continued it came under increasingly heavy fire from a number of well-sited enemy machine gun positions on the ridge, and received casualties.

[1] McKay's medals were sold by his wife around the year 1989, and his VC is now on display in the Lord Ashcroft Gallery at the Imperial War Museum, London.

The McKay VC Gymnasium is a gym facility and sports hall located across the football fields at Vimy Barracks, Catterick Garrison.

McKay was profiled in the 2006 television docudrama Victoria Cross Heroes, which included archive footage, dramatisations of his actions and an interview with his mother.

The Bloody Sunday Inquiry found that, of the soldiers who fired upon civilians, Private T (McKay) was the only one to do so with plausible cause if he was responsible for shooting Patrick Brolly and injuring him.