Dave Connell's team were using this game as their final preparation for next month's UEFA European Championship qualifiers in Slovenia having recently played Scotland twice.
A flying start got them underway with Castlebar Celtic's Jessica Nolan finishing neatly into the bottom corner following a great pass from McCarthy, who was operating in centre midfield.
McCarthy led by example as captain in every aspect of her play and fired her team further ahead three minutes into the second period when converting from a Courtney Higgins free-kick.
The UCD Waves ace got her second goal of the evening when racing onto a through ball and keeping her composure before deftly finishing into the back of the net.
Northern Ireland got one back late one, but Connell's team had done enough to secure the win and the manager was pleased with the performance of his players.
"I thought a lot of our younger players came in and did well. We played some nice passages of football, got some good goals, and overall it's been a good exercise," said Connell.
"We asked them to up the performance. We weren't really happy with the second performance against Scotland, we felt that we could have passed the ball better and we worked on that today.
"We haven't been beaten in our three games, so we look forward to our first game (in Slovenia) and we can't take anything lightly. We're expecting a tough campaign, but we'll be up for it."
Connell used this latest friendly to take a closer look at new players at U19 level, including many who featured in the recent U17 European Championships in Iceland, and he was suitably impressed.
Several players stood out in a game that Ireland dominated for large periods with McCarthy driving them forward, Sarah McKevitt providing width, and Emma Byrne impressing in midfield.
The defence was solid throughout and when they did concede a goal it came from a long-range strike. Connell will undoubtedly have plenty to ponder ahead of selecting his final squad, but that's exactly how he wants it.
Republic of Ireland: McQuillan (Maher 47); McCartan (Kealy 53), Donnelly (Best h-t), Moloney (Keenan 53), Prior (Ni Bhraonain h-t); Byrne (Lynch 53), Watters (Higgins h-t), McCarthy; McKevitt (McHale 53), Finn (Beirne 10), Nolan (Kearns h-t).
Northern Ireland: Poots; Moone, Burns, Olive, Doherty; Timoney, Myles, Carthan, McGlade; McCarron, McMaster.