Anyone can beat anyone on a given day, there are always an infinite number of variables... Imho Ireland are still number two behind the Kiwis, this defeat may just be the thing that pushes them forward to even greater heights... they will be positively frightening next week!
The difference for me between a great side/player and a good one is consistency. My biggest concern for England is that this is a one off and they lose the intensity next week. Last year in most matches they had fantastic periods followed by times where they were just making up the numbers. Scotland too, their biggest issue has always been consistency to back up some of their sublime skills. Ireland have dominated recently by being the most consistent side and playing at a high level in most games. After this weekend I think the tournament is more wide open than at the start.