McCarthy and Elliot, who are set to sign contract extensions having broken through into the first-team squad this season, have been named in the Under 21 squad to face Scotland in Galway on Tuesday 25th May, their last before the Euro qualifiers start in September.
Striker Elliott made two appearances from the bench and was tied as reserves top-scorer with Bradley Wright-Phillips on 11 goals.
Meanwhile, Flood has been named in the Under 19's for this week's UEFA Championship second phase qualifiers in Belgium.
The midfielder is already a regular member of the Under 21 squad and played in last year's World Youth Championships.