Full-back Rob Kearney has been omitted from a 45-man squad for Andy Farrell's first training session as Ireland head coach in Dublin on 22-23 December.
The 95-times capped Leinster player was a regular first-choice number 15 under previous coach Joe Schmidt.
A number of uncapped players will join a "24-hour mid-season stocktake" at the IRFU's High Performance Centre.
They include Leinster scrum-half Jamison Gibson-Park and Ulster pair Billy Burns and Tom O'Toole.
England-born Burns, who joined Ulster in 2018, qualifies for Ireland through a paternal grandparent. Prop O'Toole has become a regular in the Ulster team, scoring his first try for the province in the weekend European Cup win over Harlequins.
The other players involved who have yet to make a senior international appearance are Connacht number nine Caolin Blade and four more Leinster men - flanker Will Connors, number eights Caelan Doris and Max Deegan and back Ronan Kelleher.
Ulster prop Marty Moore, who last played for his country as part of the team which won the Six Nations title in 2015, is recalled to the extended squad, while second row Devin Toner returns after failing to make the World Cup squad.
'A better insight into players'
Along with Kearney, Jordi Murphy, Sean Cronin, Jack Carty and Kieran Marmion have also failed to make the cut, while injured duo John Ryan and Tadhg Beirne are absent.
Johnny Sexton is named despite being an injury doubt for the Six Nations opener against Scotland on 1 February.
Rory Scannell, Stuart McCloskey, Ross Byrne and Dave Kearney are among those looking to make a greater impression for a Test place under Farrell.
"The national coaches have been around the provinces watching games and having good conversations with players and the provincial coaches," said Farrell.
"We have limited opportunities across the busy provincial season to bring a broad group like this together but this 24-hour window will allow us to plant a few seeds.
"It will also give the coaches a better insight into a few players who been performing consistently well for their provinces.
"The next six weeks is the window for players to put their hands up for selection for our first squad which will be named in late January ahead of the Six Nations."
45-man Ireland squad
Connacht: Bundee Aki, Caolin Blade, Ultan Dillane, Dave Heffernan, Quinn Roux
Leinster: Ross Byrne, Will Connors, Max Deegan, Caelan Doris, Tadhg Furlong, Jamison Gibson-Park, Cian Healy, Robbie Henshaw, Dave Kearney, Ronan Kelleher, Jordan Larmour, Luke McGrath, Andrew Porter, Garry Ringrose, Rhys Ruddock, James Ryan, Jonathan Sexton, Devin Toner, Josh van der Flier
Munster: Joey Carbery, Andrew Conway, Keith Earls, Chris Farrell, Mike Haley, Dave Kilcoyne, Jean Kleyn, Conor Murray, Peter O'Mahony, Niall Scannell, Rory Scannell, CJ Stander
Ulster: Will Addison, Billy Burns, John Cooney, Iain Henderson, Rob Herring, Stuart McCloskey, Marty Moore, Tom O'Toole, Jacob Stockdale