If you stop paying the Credit union they may just let it go or they may pursue you.
If they pursue you and take you to court and you accept that you owe they money (or the court finds that you do) the court will make a judgement order against you.
This will be based on what you can afford to pay weekly or monthly. These tend to be reasonable amounts.
This process will take about 18 months, I think, (I am not very up to date on the time scale at present).
So option 1 is do nothing until you get a court summons.
Option 2 is decide yourself what you can easily afford each week, say €10 and pay that to them. This removes their incentive to take you to court.
Don't expect them to formally agree to this. They may still threaten but there are very unlikely to take any real action.
It would be nice to get their agreement to freeze the debt. I.e. not charge any further interest. If you are going down route 2, you might try this first. But in the meantime don't pay them anything, otherwise they will be under no pressure to agree.