Dragons back row forward Lydiate and the Perpignan-bound utility back Hook were both injured during Wales’ Round 2 victory over Samoa in Hamilton.
Lydiate has successfully recovered from ankle ligament damage and Hook is back to fitness after a shoulder problem.
Ospreys wing Williams, Wales’ record try scorer (55), has also fully returned from the thigh strain which kept him out of the final two Pool D clashes with Namibia and Fiji.
“Dan, James and Shane have all been passed fit to train tomorrow,” 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 (Jenkins) and Sam (Warburton) are also both fit for selection following calf cramp experienced during the end of the Fiji fixture.”
Wales name their team to face the Irish for the third time at a Rugby World Cup (they met in Pool 3 in 1987 with Wales victorious and Pool C in 1995 when the honours went to Ireland in South Africa) and the second time in Wellington (1987) on Thursday at 1.00pm local time at their new team base – the Copthorne Oriental Plaza Hotel in Wellington.
