Marcus Horan has been ruled out of Ireland's autumn internationals after undergoing an unspecified medical procedure on Friday.
Horan has not played since suffering dizziness in Munster's Heineken Cup game against Treviso on 17 October.
An IRFU statement said that Horan was expected to be fit in the New Year.
Tom Court, Stephen Ferris and Paddy Wallace are the only Ulster players in Declan Kidney's 31-man squad for next weekend's game against Australia.
Ulster trio Andrew Trimble, Chris Henry and Darren Cave are among 15 players to players to drop out of the original 39-man panel.
Peter Stringer also drops to the Ireland A panel for next Friday's game against Tonga at Ravenhill while others in the second string squad for the match in Belfast include Shane Horgan, Neil Best and Mike Ross.
Others having to be content with A squad spots are Gavin Duffy, Fergus McFadden, Sean O'Brien, Donnacha Ryan, Devin Toner and Brett Wilkinson.
Eoin Reddan earns a recall to the 31-strong main panel after missing out on last season's victorious Grand Slam campaign.
Youngster Felix Jones and his Munster colleagues Ian Dowling, Denis Hurley, Mick O'Driscoll and Niall Ronan are all named in the panel after not being original inclusions in the 39-man squad.
606: DEBATE" One of D'Arcy, Paddy Wallace or Earls to be left out, I'd wager "
Connacht's John Muldoon and London Irish captain Bob Casey are also in the same category.
Youngster Felix Jones and his Munster colleagues Ian Dowling, Denis Hurley, Mick O'Driscoll and Niall Ronan are all named in the panel after not being original inclusions in the 39-man squad.
Connacht's John Muldoon and London Irish captain Bob Casey are also in the same category.
Denis Leamy, Paddy Wallace and Tomas O'Leary sat out training on Friday because they are recovering from colds.
Friday's IRFU statement gave little away as to Horan's precise ailment.
"Following further tests this week, Horan underwent a routine medical procedure today and will undergo a period of rest over the next number of weeks," said the statement.
"It is expected that he will be available to Munster and Ireland after the New Year."
There was more positive news over Horan's Munster team-mate Jerry Flannery, who has been included after showing encouraging progress in his recovery from a calf injury.
Ireland host the Wallabies on Sunday 15 November before playing Fiji and South Africa later in the month.
Ireland squad: Tommy Bowe, Bob Casey, Tom Court, Sean Cronin, Leo Cullen, Gordon D'Arcy, Ian Dowling, Keith Earls, Stephen Ferris, Luke Fitzgerald, Jerry Flannery, John Fogarty, John Hayes, Cian Healy, Jamie Heaslip, Denis Hurley, Felix Jones, Robert Kearney, Denis Leamy, John Muldoon, Donncha O'Callaghan, Paul O'Connell, Mick O'Driscoll, Brian O'Driscoll, Ronan O'Gara, Tomas O'Leary, Eoin Reddan, Niall Ronan, Jonathan Sexton, David Wallace, Paddy Wallace.
Ireland A squad: Neil Best, Isaac Boss, Darren Cave, Gavin Duffy, Denis Fogarty, Chris Henry, Shane Horgan, Ian Humphreys, Ian Keatley, Fergus McFadden, Kevin McLaughlin, Johne Murphy, Sean O'Brien, Michael Ross, Donnacha Ryan, Peter Stringer, Devin Toner, Andrew Trimble, Dan Tuohy, Brett Wilkinson, Bryan Young, AN Other.
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