Massive Spanner
The Football Wrench
Nailed it on the first part. I'm not sure how anyone can possibly argue that the EU isn't massively beneficial to us in terms of attracting international companies.If you do, you have no understanding of economics. The reason all those companies are there is that they can have an EU headquarter in which they can avoid paying taxes and have access to a highly educated, english speaking workforce. And can bring in experts from all over Europe because of free movement.
If Ireland wasn't in the EU, American companies would need another country to base their headquarters in. Most likely, the Netherlands or Luxembourg. Irelands attractiveness lies solely in the fact it gives access to the EU while demanding low taxes.
I'd however argue that it's not the 'sole' reason. The fact that we're English speaking is a large benefit, also.