Wasps flanker OConnor, a substitute for all three matches in the championship so far, sustained a groin problem in training on Friday morning.

With little time for the openside to recover, Ireland coach Eddie OSullivan has opted to replace him with Munster lock ODriscoll.

OSullivan said: Johnny tweaked his groin and is out. Its a very minor groin strain but given the timeframe there was no chance for him to recover.

His place will be taken by Mick ODriscoll, who was in camp anyway. He knows the line-out calls and its a more seamless transition.

Ireland skipper Brian O'Driscoll has challenged his team-mates to ignite the Lansdowne Road crowd on Saturday by banishing the slow starts which have plagued their RBS 6 Nations campaign.

Victory over Scotland would set up a potential championship decider against England but with Frank Hadden's side in the grip of a stunning revival, Ireland need all the help they can get.

Italy and Wales have been dispatched in Dublin already but a subdued atmosphere shrouded both contests, partly in response to the nervous openings which have hampered the Irish.

Lions centre O'Driscoll believes his side must inspire home fans by racing out of the blocks in what is shaping up to be a memorable encounter.

"It's up to us to create the atmosphere. We need to start well and give the crowd something to cheer," he said.

"Getting the crowd behind us will feature in my team talk because sometimes they can be worth a couple of scores. We'll try and start well and get the crowd geed up.

"We've been starting a little bit slower than we wanted to. In these close encounters it's about getting hold of the ball and keeping it.

"Hopefully we'll go from the 'b' of the bang and not the 'g' and the fans will weigh in behind us after seeing some good play.''