I live in Ireland and am an Irish citizen (US citizen too, hoping to move to the US). There are several reasons I would think the US is better for entrepreneurs and startups than Ireland.
* The truly brilliant people tend to leave after high school, since there're no world class universities here (particularly for sciences).
* People seem, in my opinion, to get quite tunneled vision in terms of what they can achieve: they see Ireland as the boundary of their successes. People here tend to be unambitious.
* Labour laws in Ireland are very strict: once you hire someone, it's practically impossible to fire them.
* We are currently in a terrible economic situation, the country is bankrupt. 20 year old Irish companies have trouble getting credit, let alone startups.
* It's probably worth noting that Enterprise Ireland is generally considered quite incompetent in terms of deciding who to invest in and in general operation of their programs. Getting this grant would require, most likely, an absurd amount of hoop jumping.
* Dublin is also still a very expensive place to live.
That said, there're a lot of great things about Ireland (more specifically Dublin):
* It's a very small (and very social!) place, building a network is not hard, everyone knows everyone.
* Dublin is a great place to live.
* Very low corporation tax (12.5%)
EDIT: Formatting