England will take on Portugal, Guinea, Paraguay and Japan in their group games later this month.
Birmingham-born Grealish qualifies for the Republic through his grandparents.
Chelsea midfielder Kasey Palmer, 19, has also been included in the squad for the first time – one of five players owned by the Premier League club.
Over-age players are allowed to be selected – and midfielder Nathaniel Chalobah, on loan at Napoli from Chelsea, is also back in the squad along with Norwich City forward Nathan Redmond, 22, and Sunderland striker Duncan Watmore, 22.
The Young Lions open their campaign against Portugal on 19 May before playing Guinea four days later.
They take on Paraguay on 25 May and round off their group campaign two days later against Japan.
Goalkeepers: Angus Gunn (Manchester City), Jordan Pickford (Sunderland).
Defenders: Calum Chambers (Arsenal), Ben Chilwell (Leicester City), Brendan Galloway (Everton), Kortney Hause (Wolverhampton Wanderers), Dominic Iorfa (Wolverhampton Wanderers), Jack Stephens (Southampton), Matt Targett (Southampton).
Midfielders: Lewis Baker (Vitesse Arnhem, loan from Chelsea), Matt Grimes (Swansea City), Nathaniel Chalobah (Napoli, loan from Chelsea), Ruben Loftus-Cheek (Chelsea), John Swift (Chelsea), James Ward-Prowse (Southampton)