The 24-year-old joins the squad in place of the injured Harry Arter. The Nottingham Forest midfielder misses out through a thigh injury.
Cullen joined the squad this afternoon, after Stephen Kenny's squad trained at the FAI National Training Centre.
The Ireland squad arrived in Bratislava this evening, ahead of Thursday's UEFA Euro 2020 Play-Off Semi-Final against Slovakia.
The Ireland Squad for the upcoming games is a follows:
Goalkeepers: Darren Randolph (West Ham United), Caoimhin Kelleher (Liverpool), Mark Travers (Bournemouth).
Defenders: Matt Doherty (Tottenham Hotspur), Enda Stevens (Sheffield United), Shane Duffy (Celtic, on loan from Brighton & Hove Albion), John Egan (Sheffield United), Derrick Williams (Blackburn Rovers), Kevin Long (Burnley), Cyrus Christie (Nottingham Forest, on loan from Fulham).
Midfielders: Conor Hourihane (Aston Villa), James McCarthy (Crystal Palace), Jeff Hendrick (Newcastle United), Alan Browne (Preston North End), Jayson Molumby (Brighton & Hove Albion), Jack Byrne (Shamrock Rovers), Robbie Brady (Burnley), Josh Cullen (Anderlecht).
Forwards: Callum Robinson (West Bromwich Albion), Callum O’Dowda (Bristol City), James McClean (Stoke City), Aaron Connolly (Brighton & Hove Albion), David McGoldrick (Sheffield United), Adam Idah (Norwich City), Shane Long (Southampton)