The front row left the pitch after 17 minutes feeling dizzy and though the problem has been diagnosed, Ireland have chosen not to reveal any details.

The nature of Horan's condition has been the subject of feverish speculation and raised fears that his career might be over.

But in a statement released this afternoon, the Irish Rugby Football Union insisted the 32-year-old veteran of 66 caps would return within eight weeks.

The statement read: "It has been confirmed that Marcus will not be available for the Guinness Series games.

"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.

"It is expected that he will be available to Munster and Ireland after the New Year."

Horan's absence forced his omission from the 31-man senior squad named today for the November opener against Australia a week on Sunday.

But 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.

With Rory Best out for the season with a neck injury, Sean Cronin and John Fogarty have been retained in the senior squad to provide cover should Flannery have to withdraw.

Only Denis Leamy, Paddy Wallace and Tomas O'Leary failed to participate in training today with the trio suffering from colds.

Neither Mick O'Driscoll and Bob Casey were named in the 39-strong squad for this week's training camp in Limerick but both second rows feature among the 31 unveiled this afternoon.

Felix Jones, Ian Dowling, Denis Hurley, Niall Ronan and John Muldoon were also left out of the initial 39.

Tomas O'Leary and Eoin Reddan will battle it out for the number nine jersey with Peter Stringer included among a 22-strong Ireland 'A' squad.

Neil Best, Isaac Boss, Gavin Duffy, Shane Horgan and Andrew Trimble also feature for the second string.

Ireland face Australia a week on Sunday and also face Tests against Fiji and South Africa.

Ireland squad for Guinness Series international against Australia at Croke Park on November 15 (1500 GMT):

Forwards: B Casey (London Irish): T Court (Ulster), S Cronin (Connacht), L Cullen (Leinster), S Ferris (Ulster), J Flannery (Munster), J Fogarty (Leinster), J Hayes (Munster), C Healy (Leinster), J Heaslip (Leinster), D Leamy (Munster), J Muldoon (Connacht), D O'Callaghan (Munster), P O'Connell (Munster), M O'Driscoll (Munster), N Ronan (Munster), D Wallace (Munster).

Backs: T Bowe (Ospreys), G D'Arcy (Leinster), I Dowling (Munster) K Earls (Munster), L Fitzgerald (Leinster), D Hurley (Munster), F Jones (Munster), R Kearney (Leinster), B O'Driscoll (Leinster), R O'Gara (Munster), T O'Leary (Munster), E Reddan (Leinster), J Sexton (Leinster), P Wallace (Ulster).