There won't be many people able to gauge authenticity. The dialect snd word use in Northern Ireland is very particular (not the same as the Republic) and so is Public School speech (which varies somewhat between types of school). There won't be that many who know both.
Of course, that will apply to your readers too, so good enough may be sufficient. Fairly easy to add a few phrases to RP English to cover Public School. Northern Irish will be more subtle and there will be slight variations depending on the particular community they are from. But if they went to a Public School, they are most likely just to speak mainstream English and not Northern Irish at all - it's a very small part of Northern Irish society who went to Public Schools.
Both will depend on age of speaker of course.
Not sure that I know enough to do it, but would be happy to have a look to see if I can give any pointers about where to go.
