Only a handful of places - possibly as few as two - remain open as tensions mount ahead of O'Sullivan's self-imposed Sunday morning deadline to unveil his 30-strong squad for France.

The Ireland head coach confirmed he knows the make-up of his second row contingent, leaving the back row and back three as the only departments with outstanding issues.

A strong performance against the Scots could propel one of several fringe players into the final 30 and O'Sullivan admits he will be forced to do some soul searching on Saturday night.

He said: 'This game is very significant for those on the edge of selection.

'I always said who makes the final squad would always come down to this game in certain positions.

'The back row and back three are areas that are up for grabs. Much as we want to win the game on Saturday, it's a means to an end.

'It will play a role in the final selection for the World Cup.'

The competition is at its fiercest in the back row with O'Sullivan ready to look at four options for the one available place at Murrayfield on Saturday.

Stephen Ferris and Jamie Heaslip - who are in the starting line-up - and substitutes Alan Quinlan and Keith Gleeson will be desperate to prove they deserve the nod.

Neil Best has been picked on the blindside but should have already done enough to join Denis Leamy, David Wallace and Simon Easterby at the tournament in France.

OSullivan said: 'I have a pretty good head set on the second row situation. I wish I had the same headset on the back row.

'Carrying two back rows into the game on the bench on Saturday is a chance to see them playing. Alan can cover second row if needs be.

'It's about trying to get more access to those areas of the team where I need more information. The back row that is causing me the greatest headache.

'Like in the last World Cup, we have an embarrassment of riches in that department. I'm just giving myself one last chance to look at all the options.'