Being from NI doesn't make a blind bit of difference as far as your application's concerned; it's a part of the UK. The background checks, as far as you're concerned, will be the same.
Terms of service wise, if on a permanent commission, you'll serve until 38th birthday or you've completed 16 years service, whichever comes last.
If a graduate, you have a free option to leave after 12 years' service, but your pension rights change.
A short service commission means you're out after 12 for aircrew or 6 for ground branches, if I recall correctly.