Both players are struggling with knee injuries and will miss the game in Dublin, although Jenkins's absence could be even longer.
"Gethin is struggling and could be out for four to five weeks," said head coach Warren Gatland. "Rhys will be struggling for the first game. We're assessing him day by day.
"Gethin is a huge loss for us for potentially the first two games of the tournament. He's been playing exceptionally well for the Blues in the last month.
"Rhys was probably the find, if not the best, fly-half at the World Cup. He really does fit the type of game we want to play."
Leigh Halfpenny is expected to take over the kicking duties from Priestland.