Marnick Vermijl scored twice on his Reserves debut as United eased past Shamrock Rovers in the opening game of the Platinum One challenge.
The Belgian defender was only introduced as a half-time substitute, but bagged a quickfire double midway through the second period to secure victory for the Reds, who initially led through Tom Cleverley's penalty on the hour-mark.
Full-backs Gary Neville and Fabio both completed 90 minutes for a strong United side, but the first period was one of frustration as Rovers goalkeeper Pat Jennings produced a string of fine saves, most notably from Will Keane and Cleverley.
Jennings was replaced at half-time, however, and England youth midfielder Cleverley opened the scoring on 60 minutes as he won and converted a penalty.
In a flurry of activity, debutant Vermijl then turned in the rebound from Keane's parried shot, before ramming home a low finish after good approach play from Rodrigo Possebon.
The Reds will next face Arsenal's Reserves on Thursday. Ticket are available via www.ticketmaster.ie, priced at €20 for adults, or €15 for concessions.
United: Amos (Devlin 46); Neville, De Laet, Gill (Wootton 46), Fabio; Ajose (Vermijl 46), Norwood (Possebon 46), Eikrem (Norwood 79), Cleverley, Dudgeon (Brady 46); Keane (Ajose 82).