Southampton striker Long had hoped to make a big impression under new manager Mick McCarthy but a groin injury means that he will miss the UEFA EURO 2020 qualifiers against Gibraltar and Georgia.
O'Brien, who won the 'Three' International Goal of the Year at last Sunday's '3' FAI International Awards, will link up with the squad for training on Wednesday morning.
On Tuesday, the Ireland players used the artificial pitch at the FAI National Training Centre to prepare for the qualifier away to Gibraltar, who play on artifical surface at their Victoria Stadium.
Midfielder Conor Hourihane is due to meet up with the squad following the birth of his daughter.
Republic of Ireland Squad
Goalkeepers: Darren Randolph (Middlesbrough), Keiren Westwood (Sheffield Wednesday), Mark Travers (Bournemouth)
Defenders: Seamus Coleman (Everton), Matt Doherty (Wolverhampton Wanderers), Richard Keogh (Derby County), John Egan (Sheffield United), Shane Duffy (Brighton & Hove Albion), Kevin Long (Burnley), Enda Stevens (Sheffield United)
Midfielders: Alan Judge (Ipswich Town), Jeff Hendrick (Burnley), Conor Hourihane (Aston Villa), Harry Arter (Cardiff City*), Glenn Whelan (Aston Villa), Robbie Brady (Burnley), Josh Cullen (Charlton Athletic*), Jack Byrne (Shamrock Rovers), James McClean (Stoke City)
Forwards: Aiden O'Brien (Millwall), Sean Maguire (Preston North End), David McGoldrick (Sheffield United), James Collins (Luton Town)
*Indicates player is on loan
EURO 2020 Qualifiers
March 23: Gibraltar v Republic of Ireland
March 26: Republic of Ireland v Georgia
June 7: Denmark v Republic of Ireland
June 10: Republic of Ireland v Gibraltar
September 5: Republic of Ireland v Switzerland
October 12: Georgia v Republic of Ireland
October 15: Switzerland v Republic of Ireland
November 18: Republic of Ireland v Denmark