The 19 year old midfielder had represented the Republic at under 21 international level and chose to play for them at full international level ahead of Scotland.
McCarthy has an Irish grandfather and so chose to play for Giovanni Trapattoni's side. He made his debut as a second half subsitutute last night against Brazil.
The Irish lost the game 2-0 but McCarthy enjoyed the experience. He said: "I was delighted just to get on the park and make my debut. That's it put to bed now [the Scotland speculation] and from day one I wasn't going to change anyway.
"I just need to keep working hard with my club and hopefully get more international recognition. I was a bit nervous about being the new boy in the squad but everyone made me welcome.
"They are a good bunch of lads and I am delighted to make my debut. It is brilliant to be honest to make your debut against top opposition like Brazil and the only disappointment was the result."
