“Killed in action” (on the mural) or “on active service” (on the plaque) often means killed by a bomb exploding prematurely, as in the case of Finbarr McKenna, who died in Crocus Street intending to attack the RUC station on the Springfield Road at Violet Street. Here is an account of McKenna’s death from a British soldier. Lost Lives estimates that as many as 163 volunteers (9% of the total killed by the IRA) died from premature explosions. Footage of McKenna’s funeral appears in the (Sınn Féın-produced) account of Larry Marley’s funeral.
The plaque dates back to at least 2004.
Click image to enlarge
Copyright © 2018 Seosamh Mac Coılle
X04992 In memory of IRA Volunteer Finbarr McKenna who died on active service in Crocus St. 2nd May 1987. Erected by the Greater Clonard Ex-Prisoners Association.
Extramural Activity 
The soldier’s version of the events surrounding Finbarr McKenna’s death, which you have linked, are largely fabricated. He did not suffer the kinds of injuries described, and the accounts of the actions of dogs are, therefore, impossible. I know because I was on the scene in its aftermath.
I was there from the start. He was dead at the sence, due to the fact he made a mistake. He threw a homemade bomb at RUC Springfield police station, unfortunately he was unable to decide what to throw and the bomb when off killing him and injuring 20 innocent people. He was dead before I could treat him. As a paramedic, he was dead instantly, but managed to injure a pregnant woman and children. No soldiers were involved.