Are you buying a house where you are moving to?
How much will it cost?
How will you fund it?
On the face of it, your former home is a great investment
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Investment €100k( €300k - €200k)
If you pay the €100k off your new mortgage, you will save about €2,500 in interest, so if you don't mind the hassle and risk, keeping the property is the right idea.
Another reason for keeping it, any increase in value up to €360k will be free of CGT. And after that the CGT will be reduced as it was your Principal Private Residence for such a long time.
Alternative
Depending on which lender you have the mortgage with, you should be able to port the tracker to your new home.
That is less valuable these days as the new rate would be c. 1.8% - still below what you can borrow at, but as big a gap as before.