He moved to Newcastle Falcons where he established himself as a regular member of their first team.
The 29-year-old subsequently moved to Connacht and his ability to play either at lock or blindside flanker has made him a valuable asset and brought him into contention for the Ireland senior squad.
All his friends in the Exiles wish him well for his international debut.
The player himself recognises how big a moment it will be when he pulls on the green jersey for Saturday's Test match.
"It is a really big opportunity and I'm really looking forward to it. After a good few weeks of pre-season and hard work, getting the chance to play is the easy part, I just want to put a good shift in and hopefully impress all the coaches," said McCarthy, whose grandparents hail from the Mayo town of Belmullet.
"I've always felt I was good enough, I've always backed myself and I've always worked hard, and I'm just delighted to get a chance.
"I'm very grateful to Connacht. There's great coaches there. Dan McFarland has really helped me in the forwards there and Eric (Elwood)."
Ireland head coach Declan Kidney has admired McCarthy's progress with the province, with his ball-handling ability, athleticism and eagerness for work being obvious plusses.
"He's probably a nightmare to train with in pre-season because he's always out in front. Mike's very athletic, he'd be one of the top trainers," explained Kidney.
"You look at Connacht's lineout stats - they've been excellent, though obviously that comes down to more than a jumper.
"He's got good ball-handling, and after that we won't put any more helium into the balloon because that will only make Saturday more difficult for him. He's alright. He's got a bit of an accent, but sure we'll get over it!"