Coach Eddie O'Sullivan had planned to recall the Leinster full-back after he missed Ireland's 37-13 win over Italy a fortnight ago and move Murphy onto the wing.
But Dempsey, who has been suffering from the same injury since coming off during Leinster's Heineken Cup clash with Bristol two months ago, complained of soreness and was once more withdrawn.
With Murphy filling the full-back void for the second successive match, Munster winger John Kelly gets the chance to win his ninth cap and team-mate Rob Henderson will be on the bench.
The late alteration means the only change to the team which won in Rome is in the front row, where Marcus Horan takes the place of Reggie Corrigan, who broke his wrist in that match.
"It's a pity, but it's no great drama," said O'Sullivan.
"It was an easy enough decision. We'll have the same back division that played quite well against Italy.
"We kept John and Rob in the camp because these things can happen. It doesn't disrupt us too much."