Originally Posted by
stut
Your main options are via EWR (CO), or LHR (BD/EI). There are other low-cost options from the UK mainland, but you'll need to re-check (with no guaranteed connections), and may be subject to harsh baggage restrictions and/or fees.
As an alternative, if you have an easy direct flight to Dublin, there is a bus from Dublin airport direct to Belfast, taking 2h25. Note that, though, transiting via Ireland, it does get a little complex as far as immigration is concerned.
As
moremilesplease has said there is no border stop going from the republic of Ireland to northern Ireland. The road from Dublin to the north is very good, motorway most of the way so via Dublin is a good option for you.