What is your current tracker rate. If your tracker is ECB +0.5%, then it's clearly worth keeping as the new rate would be ECB +1.5%.
But if your rate is ECB +1.25%, then while it's still worth keeping, ECB +2.25% is not as valuable.
How many years are left on your mortgage? 20 years to go is well worth keeping. 5 years left is less so.
It is very difficult to buy and sell in the one go. Possible, but difficult.
If the builder is flexible, then it would be possible. But if you agree to sell your house in June with a closing date of October and the new build is not ready, what happens then?
The usual procedure for keeping a tracker is as follows.
1) Sell your own house and pay off the tracker.
2) Buy the new house within 6 months and get a new mortgage with the increased tracker rate.
I don't know the terms of the BoI mover's mortgage, but it's likely to require you to sell first and buy within 6 months.
If on the other hand you buy first, it's unlikely that they will allow you to port the tracker.
If you have the means to buy now without selling, have you considered keeping your existing home as an investment? I am not recommending it. I think you should just do the maths and see if it's worth doing.
It saves a lot of hassle. You buy the new house when it's ready. You take your time in moving. And then you let out your old house when it's ready.
Brendan