The 16-year-old winger joins from North Dublin club St Kevins Boys after Sir Alex Ferguson beat off competition from several rival Premier League sides chasing the youngster, who will move to United later this year.
St Kevins Football Director Ken O'Donoghue says United had beaten all of their main rivals to the addition of the Republic of Ireland Under 16 player, who he likened to a current Red Devils superstar.
"All the top clubs were after Robbie," O'Donoghue told setantasports.com.
"You could name any one of fifteen of the top Premier League teams and we've had invitations from them.
"He 's a lively wide player. A lot of people have compared him to Liam Brady, but he's more a Ryan Giggs type of player, a fast out and out winger."