Firstly, I'm not in a position to give you legal advice, and from your post, you don't need it.
Secondly, I have my own moral base, it's mine and I won't be imposing it on anyone. The same can be said for you - it's your private concern.
Cars parked.
You need to write to your Financial Instution (FI) and ask them for all relevant personal information about you under the Data Protection Acts 1988 and 2003 dataprotection-dot-ie ..click .. 'for individuals'.... 'know your rights'. As well as giving you access to all off your personal information (documents signed by you) it can include 'cctv' images i.e. if you know when you went into your FI - as in the date on the Credit Agreement (CA). Everyone can access their own personal information, your Father can (& should) do the same.
You should be given copies of your CA and older agreement. Check if they are look identical (hold one over the other up to a window). If you think the do engage the services of a respected forensic expert (legal circles should point you to 1 individual in Eire, you won't find him on google).
Once you get your personal information from your FI and you think you have a forgery on your hands, make an official complaint to your FI. Do not do this before your have the evidence in your hands. Information about this is at nca-dot-ie/nca/financialcomplaints (my 1st post here so can't post url's, workaround OK moderator?)
Stop communicating verbally with your FI.
It is your FI's right to enforce their rights as they see fit, i.e. legal. That's their decision, you will not be able to influence this in any way, nor should you. If they choose this course, as is their right, it sounds like you MAY (caveat, read my 1st sentence) have a possible defense under the Consumer Credit Act, and the potential forgery. Get legal advise at that point, the loan sum is relatively small so the legal expense won't be huge. As far as I am aware, you do not have any counterclaim against your FI, unjust enrichment etc.
If you don't pay, don't be under any illusions that they won't try and enforce their actions. Find a copy of the March 2011 'financial measures programme' online, go to page 24 under the heading 'asset quality review' and contemplate the words '...potential ability to remediate...'
Yes I know, different size and class of loans etc, but... think institutional mindset.
This is not advise or a recommenddation per se... but if I were in your shoes I would be getting some hard cold facts as opposed to "I'm, fairly sure" etc. you've got nothing until then.