Shamrock Rovers duo Graham Burke and Aaron McEneff (pictured above) come into the squad as well as U-21 duo Troy Parrott and Jack Taylor.
The Millwall forward, who is on loan from Tottenham Hotspur, and the Peterborough United midfielder will now link up with the senior squad.
Jeff Hendrick has been ruled out of the Bulgaria through suspension due to his red card against Wales. Jayson Molumby has also been suspended as he picked up his second yellow card of the UEFA Nations League campaign.
Forward Adam Idah has also been ruled out of the Bulgaria match through injury and has returned to his club.
Republic of Ireland squad
Goalkeepers: Darren Randolph (West Ham United), Caoimhín Kelleher (Liverpool), Mark Travers (AFC Bournemouth)
Defenders: Kevin Long (Burnley), Shane Duffy (Celtic), Dara O'Shea (West Bromwich Albion), Cyrus Christie (Nottingham Forest), Ciaran Clark (Newcastle United), Darragh Lenihan (Blackburn Rovers), Ryan Manning (Swansea City).
Midfielders: Conor Hourihane (Aston Villa), Robbie Brady (Burnley), Jack Byrne (Shamrock Rovers), Jason Knight (Derby County), Josh Cullen (Anderlecht), Aaron McEneff (Shamrock Rovers), Jack Taylor (Peterborough United).
Attackers: James Collins (Luton Town), Ronan Curtis (Portsmouth), Callum O'Dowda (Bristol City), Daryl Horgan (Wycombe Wanderers), Sean Maguire (Preston North End), Graham Burke (Shamrock Rovers), Troy Parrott (Millwall, on loan from Tottenham Hotspur).