12 Dec, 14:42
Prop Cian Healy is expected to miss the start of the 2014 RBS 6 Nations after undergoing surgery on his injured ankle on Wednesday.
"All that matters is Friday night. It's a 'winner takes all' situation," Sheahan said when assessing how Ireland's World Cup campaign has gone.
"There is a bit of fear that we haven't been firing on all cylinders yet but fear can be good and can bring the best out of us come Friday night. The one positive I would take out of that match last week (against Georgia) is that we didn't lose it.
"I think the boys did extremely well when the Georgians got their dander up in the last few minutes and threw everything at us and it could have gone horribly wrong," he added.
"We got the four points out of it - okay, we would have hoped for five but that's it and it's now forgotten. We've a good squad here, one that came close to winning the Grand Slam and you don't become a bad team in the space of a couple of months. It's not as if guys are out drinking and smoking or whatever."
Sheahan was one of five players in the Irish squad at the last World Cup who failed to get any game-time at the tournament in Australia. Now 31, he knows time is not on his side but having been promoted due to a combination of hard graft and an injury to Rory Best, he is "delighted" to be in the matchday 22 to face France.
"I was disappointed for the first match or two but it's fantastic to get the chance now. There was a group of eight of us who trained really hard with (fitness and conditioning coach) Mike McGurn. I was rooming with Alan Quinlan, he kept me going and vice versa, and I've now got the break and I'm delighted."
Sheahan insisted that squad morale is good despite Ireland's failure to live up to their billing as potential World Cup winners in their opening two Pool D matches.
"There's a good feeling in the squad in spite of what's happened so far. Because we were perceived to be that much stronger than Namibia and Georgia, I think we chased the bonus point too early," he admitted.
"We were trying things off first and second phase that you wouldn't normally do and they didn't come off. Basically all of a sudden the crowd get behind the underdogs and you're in a dogfight."
Shrugging off comments that Ireland are "in crisis" as they approach one of the biggest World Cup games in their history, Sheahan added: "In crisis? Far from it, our backs are to the wall a small bit, there's no hiding away from that and guys aren't hiding away from that but we want a big performance against the French.
"Obviously there's a bit of nervous tension and that little bit of fear that I spoke about, which is no harm going into a big game. You're going into the lion's den and there's no place for passengers. I think the morale is building and it's going to come right on Friday night."