The 17 year-old winger has already featured for United's reserves.
He told manutd.com: "I was playing for St Kevins Boys Club back home in Dublin and I was playing for my county team as well. I got invited over to United and a few other clubs were interested, but this was always the place I wanted to be.
"It's brilliant. Being here is the sort of thing you dream about as a young kid. Playing football is all I've ever wanted to do. If I didn't play football I don't know what I'd do. Playing for United is unbelievable, more so when you actually sit down and think about how fortunate you are. Most of my friends are fans so they were delighted that I signed, especially if I get them tickets!"