@Puddle duck
You should start a separate thread on the trading up question. This is not the thread for such a complex issue.
With a margin of 1.15%, you will soon be paying 4.15%
You can fix for 2 years at 2.95% or 4 years at 3.1% or 7 years at 3.55%.
If you are definitely keeping the house and mortgage, fixing for 4 years would be clear.
But as you might sell the €500k house, it's unclear. If you fix, you could face an early repayment penalty.
So you have to decide, quickly, how long you might have the €500k house.
If you genuinely don't know and the house is not definitely being sold but might be sold in the next couple of years, I would fix for 2 years.
A further problem is that if you keep it as a rental, AIB might well charge you buy to let mortgage rates when the fixed rate is up.
On reflection, you need to make some decisions on your housing and investment plans and the mortgage decisions will follow that.
In the meantime, stay on the tracker.
Brendan