And playing in his third World Cup, Williams reckons the youngsters in the squad have provided an extra boost of confidence.
"There is a lot of self belief in this squad and it's the most mentally strong side that I have been involved in," he said.
"We have always been able to play rugby and throw the ball about but now we are clever with our approach and are much stronger mentally.
"Look at the youngsters coming through, they are full of confidence and perhaps that has given us more senior guys a bit of a lift.
"It is going to be a massive game for us on Saturday but we know if we can play our best rugby we can with the game and it is as simple as that.
"We have trained well and worked our socks of all summer and we know we can beat the world's best sides.
"There is always a fear of not being successful and if we lose on Saturday we would not have been as successful as we would have wanted to be, but right now we are just excited.
"We know what is on offer and what could be around the corner if we perform, but we also know what is waiting for us if we lose.
"I have a lot of respect for this Irish side. They know how to win big games, are very experienced and have caused us problems over the part few years."