We did this in 07 (thank the stars !!) when we moved down the country. In effect, you have 2 transactions to complete
Firstly you have to sell you existing house and ensure that either the sale price pays off the mortgage or you can fund any shortfall yourself
You then need to buy another house and this will be done via a new mortgage on the new house (if a mortgage is needed).
You'll also need to consider funding stamp duty (if required), auctioneer and solicitors fees.
It's doubtful if you'll get a tracker on the new mortgage
It's likely that in addition to stamp duty you also need to have enough from the sale of your house or from savings for at least a 10% deposit.
In the current climate you should do 3 things
Firstly, ask a couple of estate agents for a valuation of your existing property, knock 10-15% off that since it's a buyers market and see will that clear your mortgage
Talk to your bank and see what mortgage you can get for the new house and who is prepared to lend
Look around for the new property
Realistically, if you buy a house for €250k, to cover stamp duty/deposit/fees etc you would need around €40-€50k