'Ireland seem to be going from strength to strength, with threats from everywhere,' said Williams.
The Cardiff player warned: "We will have to be on song or it could be a difficult day.'
Ireland announce squad.
Williams has established himself as one of Steve Hansen's regular players, and knows that in the current climate, every performance is scrutinised by a Welsh public that expect success.
Wales have failed to register a win in three RBS Six Nations games, and Williams has warned that for that trend to be reversed, Wales must get off to a good start.
'You can not afford to give an international side with the class Ireland have the sort of start that we gave Scotland,' he added.
'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.
'We can't afford to do that again."
Williams has scored seven tries in 25 appearances for Wales.