I got married in a foreign country last year (both of us irish) and upon return was told when I phoned the registry office that there is no requirement (or method) of registering a foreign marraige in Ireland. So long as it is a recognised (and legal) civil marraige in the country in which you marry it is recognised in Ireland.
I would think as far as pre nup and divorces go, if it comes to divorce you will have to apply for the divorce in the country in which you live, so if you live there it would be handled there under US law.
I would see an Irish family law solicitor if I were you.