Royal Munster Fusiliers, WWI

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Hi....I'm looking for information on a man named John MacMahon(McMahon) he may have been a Royal Munster Fusilier...he also had a friend with the last name Munyard...John MacMahon was my great grandfather and I'm trying to find anything I can about him for my family tree....He was with Munyard in Chateau d'Oex, Vaud, Switzerland in the years 1916-1918 at least....and thats really all I know about him.

thanks