Furlong said after Japan's win over Scotland finally sorted the tournament quarterfinalists that while Ireland and the Lions may have beaten the All Blacks that counted for nothing this time around.
"Rugby evolves quickly and the game moves on, stuff changes, systems change," he said.
"I suppose it gives us confidence that we have had results in the past but at the same time it doesn't guarantee anything.
"It doesn't mean they're going to rock up and play with the same plan as before.
"They are so dangerous, you can't switch off," he said.
Getting beyond the quarterfinal, something Ireland has never achieved at a World Cup, required a basic approach.
"Just working really hard and staying switched on mentally is crucial because they can make something out of nothing with some of the players they have," he said.
"I suppose there's an element of just being physical.
"It's the same as any rugby game you play but as a front-five forward, it's all about being as physical as you can and hopefully coming out on top," he said.
Coach Joe Schmidt said playing the All Blacks would be a daunting prospect.
"I feel that you've got to be at your very, very best to have a chance and the All Blacks are a sort of team that you could play at your best and still not get the result.
"Because they weren't number one in the world for ten years and they're not back-to-back World Cup winners for no reason.
"They're that because of the personnel they have, because of the culture they have and the performances they manage to put together on a very regular basis.
"So for us it would be, to a degree, a mountain to climb. Obviously the whole thing gets steeper next week," he said.
Assistant coach Andy Farrell, who will take over the head coach role after the World Cup, said Ireland would assess their win over Samoa and then set themselves up for the weekend.
"I don't think form really comes into it next week," he said.
While it was a good performance to beat Samoa he didn't know if that would be good enough in the playoffs.
"All you've got to do in a quarterfinal is win.
"Of course, form on the day matters, whether you can carry that on from a year ago, six months ago or last week, it all becomes a little bit irrelevant in the end," he said.
"At the end of the day, the game takes its own course and you've got to manage the circumstances that you're in at that moment in time."