Its been a while but when I did mine it was just a "tell us about yourself" type interview, where the examiner clearly wanted to give the applicant the opportunity to show off any Irish they had. (Not so much that I would have gotten away with a memorised speech though.)
To prepare yourself I recommend taking any opportunity you can to speak Irish- if you haven't used it for a while you may be a bit tongue tied and come across knowing less than you do.
If you can get TG4 where you are, watch that too, if necessary chosing a programme with English subtitles, to tune your ear in. Listen to the nuacht as well, so you recognise the Irish vocabulary associated with topical issues.
You mention getting through to the next round- are you sure that the Irish exam is part of the first stage? It may be that the first stage is verbal and numerical reasoning- worth checking out.
Good luck- Castlebar is a great town!