My fiancé and I both live in the USA. I am a citizen here, and he is a UK citizen but also a legal alien in the USA (holder of a resident alien card for 17 years now). We want to get married in Northern Ireland where his entire family is. What steps do we need to take to make our marriage legally recognized in the USA? What forms do we need, etc. Neither one of us are trying to change our citizenship--I will remain US citizen and he will remain a citizen of the UK.

In Ireland you will be married by a JP, same as here, or a priest or minister. You only need your US passport.