I moved my mortgage from AIB to NIB in 2007. With AIB my parents were guarantors but when I moved to NIB there was no mention of this and I didn't sign anything about it with NIB.
Does the guarantor agreement also get transferred to NIB as part of the transfer?
It looks like you took out a new mortgage with AIB and that didn't require a guarantee. Check with your solicitor and the mortgage department of your bank. Also you need to check the original guarantee with NIB and make sure that you read it. These things can have all kinds of clauses to continue the guarantee for all loans etc.