Dublin's startup scene is at a much earlier stage, so there certainly isn't the same sort of community, talent, mentorship etc.
Raising money would also be more challenging, since there are only a hand full of VC's.
Sounds a little trivial, but it is worth saying that there is a really strong sense of pessimism here at the moment (because we're basically bankrupt). This is actually rather overwhelming and hard to escape from.
I also agree with the point regarding ambition. There isn't the same rampant enthusiasm here to actually build things and achieve something. So its harder to find like-minded co-founders... people who are willing to throw caution to the wind, work incredibly hard and just go for it.
Having said all those things, its undeniable that there is definitely something really positive happening in Dublin at the moment. This is the second incubator fund that's been put together (the first closed in jan of this year). Also take a look at the dublin web summit... http://www.dublinwebsummit.com/
Dogpatch has also announced that they are opening an incubator in Dublin, and DCU's incubator has just joined the Techstars network.
There are also tonnes of great hackers in Dublin, many of whom are world class. The problem is that most are comfortably tied up in their jobs at google or wherever. During the last 10 years here, the game plan was this... get a good job, leverage yourself up to the tonsils to buy a house and watch as it increased in value by 20% year on year. They buy another. People were becoming paper millionaires in their 20's, so there was no incentive to try something else. Things are different now and I think that the startup bug is going to catch pretty soon. All it takes is one reasonably successful exit and the media will be all over it. Then you'll see some great talent being shook loose.
It's not the valley, that's for sure, but for Europe I think its a great option.