After 10 cagey opening minutes, Oliver McBurnie finished well following a neat move. Within minutes, it was two, Demetri Mitchell and McBurnie forcing a stunning double save from the Cherry Orchard goalkeeper but Joe Riley was on hand to ram home the rebound from six yards. McBurney grabbed his second from close range after the visiting defence failed to deal with a low Andreas Pereira cross.
With victory almost assured by that first-half burst, Paul McGuinness granted opportunities to members of the squad and, despite Conor Foley's well-taken goal six minutes from the time, the Dubliners were unable to recover.
United have now reached the final of the competition for a record 11th time and will meet Co Tyrone on Friday with the trophy up for grabs.
United: J.Pereira; Reid (Dick 62), Tuanzebe, Borthwick-Jackson, Thompson; Riley (Scott 62), Willock (Rashford 45), A.Pereira, Mitchell; Croskery (McTominay 45), McBurnie (Nicholas 55).