I think it's very unlikely that your existing lender, BoI, I presume, will give you €80k to pay off a family loan, irrespective of the purpose of that loan.
It's possible that another lender would refinance the whole mortgage. The most likely candidate being Pepper, but that would be at a higher than market rate.
I suggest that you apply to Bank of Ireland now for the loan. They will refuse you. You could then ask them if they would refinance it after 18 months. When they see the reason you initially borrowed the money, they might do so.
Brendan