The team is due to fly out to Antalya in Turkey for a friendly on Monday, following an almost complete squad meeting on Sunday.
The six players who have been dropped from the original 30-man line-up, due to injury, are defenders John Egan and Greg Cunningham, midfielders Harry Arter, Liam Kelly and goalkeepers Rob Elliot and Keiren Westwood.
Bradford goalkeeper Colin Doyle will join the squad after his League One match against Doncaster on Monday.
O'Neill has included eight previously uncapped players in his squad for the friendly in an attempt to restructure the team after missing out on World Cup finals.
The full Republic of Ireland squad is listed below:
Goalkeepers - Colin Doyle [Bradford], Kieran O'Hara [Manchester United], Aaron McCarey [Ross County]
Defenders - Seamus Coleman [Everton], Shane Duffy [Brighton], Ciaran Clark [Newcastle], Cyrus Christie [Fulham], Matt Doherty [Wolves], Declan Rice [West Ham], Kevin Long [Burnley], Alex Pearce [Derby], Derrick Williams [Blackburn], Enda Stevens [Sheffield United]
Midfielders - James McClean [West Brom], Alan Judge [Brentford], Conor Hourihane [Aston Villa], Jeff Hendrick [Burnley], David Meyler [Hull], Eunan O'Kane [Leeds], Alan Browne [Preston], Daryl Horgan [Preston]
Forwards - Shane Long [Southampton], Darragh Lenihan [Blackburn], Scott Hogan [Aston Villa], Sean Maguire [Preston]