I don't think Irish neutrality is guaranteed. But (speaking as a Europhile, but not a one-nation-of-Europe-phile) the Irish people feel that while they're very grateful for all Europe has given them, they're not grateful enough to trade away their nation for it.

The objections aren't to enlargement, in anyone I've talked to. They're to the two-tier Europe that's being attempted, where countries like Germany will have a permanent seat on the Council of Ministers, while small countries like Ireland (or Poland, or Croatia, when they come in) will not.