Lions duo Williams and Hook have been out with thigh and shoulder problems respectively, while blindside flanker Lydiate was suffering with ankle ligament damage.
Hook, Lydiate and Williams haven't played since the hard-fought win over Samoa on September 18, missing the convincing victories over Namibia and Fiji as Wales secured a second-place finish in Pool D.
Squad skipper Sam Warburton and Lions prop Gethin Jenkins were also injury concerns earlier this week but the Cardiff Blues pair have been given the all clear after minor calf issues against the Fijians.
"Dan, James and Shane have all been passed fit to train," said WRU national medical performance manager Prav Mathema.
"Pending any adverse reaction we expect them all to be available for selection against Ireland this weekend.
"Gethin and Sam are also both fit for selection following calf cramp experienced during the end of the Fiji fixture."
While Jenkins and Warburton look certain to start against in-form Ireland, the fate of the other three injury victims remains less clear.
Lydiate, Hook and Williams were all in head coach Warren Gatland's first-choice XV at the beginning of the tournament but the form of their replacements means they may be overlooked in the last eight.
Wings George North and Leigh Halfpenny have shone in Williams' absence, while Hook's spot at full back is under threat from Lee Byrne and Halfpenny. Hook has made it clear that his preferred position is fly-half, but Rhys Priestand appears to have made that spot his own for now, with Jamie Roberts, Scott Williams and Jonathan Davies all impressing in the centres.
Lydiate's slot has been filled by veteran back rower Ryan Jones, with Andy Powell coming off the bench, but the Dragons youngster looks likely to be recalled straight away given the impact he has had on the side over the past season.
Gatland is due to name his matchday 22 at 1am BST on Thursday.