You could also argue that Ireland was in the same economic state back in 73 as some of the new eatern European states are now; I would even go as far as saying that some of the new memebers are at a more advanced stage of economic development than Ireland was 30 years ago.
To say though, that the union could fall apart because of the diversity of cultures, economies etc. is a little bit unfounded, in my opinion. The recent French no vote, and the very likely Dutch no vote on the constitution, says nothing more than that it's too early to be doing so. I think that the whole media and political fuss about this being the end of a EU constitution is total BS. The no votes are more an inconvinience, as it means that the whole process of renegotiating between all states needs to start again; which it will.
I do agree on the point though, that the last enlargenment was too big/fast. 10 new memeber states is an awful lot, especially with the 2 largest economies, Germany and France, in such limbo; it was a big risk and we'll have to wait and see what the outcome will be.