Why do you think that is? I would have thought it's primarily because of the economic benefits that Ireland got after joining?

That's one aspect. There are lots of new motorways in Ireland (and many poor EU states) which are mostly financied by the EU. Ireland as a small country can't generate a big local market, free trade with the rest of Europe is good. Many liberal people support it because it's forced through a lot of liberal law changes. And, we're either align ourselves economically with Britain or Europe. Nothing like a bit of anti-British sentiment.