He will join Head Coach Jim Crawford as an Assistant Coach.
Paul, who will continue his role at United as lead coach of the Academy's Under-15s, has his UEFA A Licence and was accepted onto the Football Association of Ireland’s UEFA Pro Licence course earlier this year.
He will join up with the Ireland Under-21 players and staff ahead of next week's games against Turkey (Friday 8 September) and San Marino (Tuesday 12 September).
The former United player fills the position vacated by fellow ex-Red John O'Shea, who is now part of the Irish senior team backroom staff.