Clifford will join up with the seniors after playing for the under-21s against Austria in Sligo on Tuesday.
Ireland coach Giovanni Trapattoni needs the 19-year-old to provide cover in midfield having seen a string of players pull out of his squad due to injury.
Stoke pair Jon Walters and Marc Wilson are the latest, with James McCarthy, Seamus Coleman, Keith Fahey and Damien Delaney - who has been replaced by Ciaran Clark - some of the other most recent withdrawals.
Wolves striker Kevin Doyle is set to feature against the Croats having received his first call-up since sustaining knee ligament damage in March, while fellow forward Shane Long, who was reported to be in talks with West Brom about a move from Reading, also trained with the group at Malahide on Tuesday morning.
Source: PA
Source: PA