United have 3 players in the U18s squad and 1 in the U19s.
18-year-old centre-back Regan Poole is hopeful of earning a loan move into the Football League next season.
Both Barlow and O'Connor played in their country's final group game, and will face each other in the U17 European Championships quarter-finals.
Three minutes after coming as a sub, Aidan Barlow netted on his England U17 debut.
Aidan Barlow (England) and Lee O'Connor (Republic of Ireland) have been called up for the Under-17 European Championships in Croatia.