We were talking about companies moving production there to take advantage of lower wages, weren't we? So presumably the jobs are there. If they aren't, like I said, investment is very important, and the EU does that to some extent. It's worked very well for some countries, but not (yet?) so much for some others.
We'll see how it plays out, but in general, I'd say that if goods, services and money can cross borders with no problems, people should be able to do the same.
Sure, if there's demand for jobs. But when you can't hire people, how do you keep your company running? And how do you compete without the appropriate number of people? Not being able to hire has an effect on the economy as well, and companies will likely be outcompeted by those that can hire. And if the company goes away, who's trying to hire people, driving up the wages?
I believe that's why you'll often see a combination of high unemployment, low wages and high emigration.
And sure, investments will work, but I'm not sure if you can replicate Ireland's success to e.g. Portugal or Spain. You can't turn every country into a tax-scheme for multinational corporations with a friendly regulatory environment regarding privacy, we don't have enough Googles and Facebooks for that, and transitioning that into continuing success isn't as easy when your country isn't English speaking.