It's easy to set up a new company, the overhead is minimal and there are lots of financial and other supports available from government agencies like Enterprise Ireland:
http://www.enterprise-ireland.com/en/
We have a very low corporate tax rate and the government recently introduced R&D tax credits to encourage innovation:
http://www.bdolimerick.ie/department.php?pg=/Research%20and%...
Not to mention a very strong academic community, plenty of funding available for academic collaboration and the presence of a huge number of prominent US tech companies including Google, Facebook, Twitter, eBay, PayPal...
http://thenextweb.com/insider/2011/11/26/what-attracts-big-t...
The Irish startup visa program has recently been amended to make it easier to qualify:
http://founderware.co/ireland/ireland-launches-startup-visa-...
You now only need to invest €70k ($91k USD) to be granted residency. This is down from the old requirement of €300k.
Taxes here are quite reasonable. Corporations are charged 3.1x less tax here:
http://www.americanthinker.com/2009/03/us_companies_pay_the_...
On average, UK taxes are generally higher:
http://www.nationmaster.com/compare/Ireland/United-Kingdom/T...
And it compares favorably with the US as well:
http://www.nationmaster.com/compare/Ireland/United-States/Ta...