Hooker Sheehan and back-rower Conan have both been struggling with foot injuries, while prop Kilcoyne has been managing a hamstring issue.
"A few guys will not be involved at the weekend," Ireland assistant coach Paul O'Connell was quoted as saying by the BBC on Tuesday referring to Sheehan and Conan.
"David Kilcoyne as well is struggling a little bit but should be OK next week."
O'Connell reported that, apart from those injuries, the team are in good health ahead of the game.
Ireland enter the competition as the top-ranked team in the world.
They take on Romania in Bordeaux in Pool B and they also face Tonga, South Africa and Scotland.
The 10th edition of the World Cup starts on Friday with hosts France facing New Zealand and concludes on October 28th with the final to be played at Stade De France.
