financially it depends how long is left on your mortgage, your mortgage rate, and the rate you would be offered on any loan. a mortgage top up may seem cheaper, as it would likely be a much lower monthly commitment, but if you have 15 years left on your mortgage then remember that you will be paying interest on those funds for the next 15 years, which would amount to a considerable hike to the amount you pay back overall. no idea if ub would let you have a top-up on a tracker though...
try doing a few calculations on the loan calculator on itsyourmoney.ie (no affiliation)
for example, a personal loan of €20000 over 5 years at 12% (rough average) is €438.71 a month and would cost €6322.84 in interest.
the same amount over 15 years at 5% as a guesstimate for a mortgage top-up would cost you €157 a month, but cost €8260.75 in interest, so you pay more in the long-run, but considerably less a month.
all in all depends on your personal financial situation really...