"You cannot afford to give an international side the sort of start that we gave Scotland," Williams said.
"We didn't impose ourselves on Scotland and basically we didn't turn up in the first half, although our second-half performance was a lot better.
"Ireland seem to be going from strength to strength, with threats from everywhere, and we will have to be on song or it could be a difficult day."
Coach Steve Hansen is due to announce a starting line-up with recalls expected for hooker Jonathan Humphreys, as long as he recovers from a shoulder injury, and deposed captain Colin Charvis in the back-row.
Pontypridd prop Gethin Jenkins is also considered a strong candidate, with tighthead Ben Evans under pressure, while Williams can expect a switch from wing to full-back as replacement for knee-injury victim Kevin Morgan.
If Williams is moved, then either Llanelli wing Mark Jones or Cardiff's Craig Morgan would be favourites to secure a starting place.