The famous British Army regiment The Connaught Rangers had barracks at Boyle in a house called Kings House. This building is still there and is now a museum. There are a number of sites devoted to the Connaught Rangers such as:
In more recent time the actress Maureen O'Sullivan was born in Boyle where her father was an officer in the Rangers.
I believe there are full army records available at Kew Gardens in London and you should be able to trace your ancestor there. There is a website but not sure of it offhand. Perhaps someone else can help there.
: : My ancestor Edward Jacques was born in 1815 in Boyle, Roscommon.His father was a soldier. Does anyone know anything about the army in Boyle -perhaps recruiting. I think he was in the 3rd.regiment of foot guards.