
To the best of my knowledge, the rules are that there aren't any
The vast majority of trains in NI call at all stops en route (the main exceptions being the Enterprise to Dublin and some peak-time Belfast-Bangor services) so it doesn't normally become an issue.
There are far fewer ticketing anomalies in NI due to the whole system being run by the one company, however. What is it you were planning on doing?
Is that for a day return or a single?