It's very hard to know what to do.
If you have an Ulster Bank cheap tracker, you can transfer it to another property. The availability of such a cheap loan would be a deciding factor for buying now.
Do the rent vs. buy calculations. For example, if you are planning to buy a €300k house, it will cost you around €13,500 a year in interest.
How much would it cost to rent such a house? If it's a lot more, then you should consider buying it.
Of course, it's not enough to compare the existing cost of renting vs. buying. You also have to anticipate how interest rates and rents are going to move. While both are very difficult to forecast, interest rates are artificially low at the moment and should return to more normal long term averages in the medium term.
If you sell your home, you will have to use your savings to discharge the negative equity. You won't have a choice in that.
If the bank gives you a negative equity mortgage, you could hold onto your savings. You should do so for the time being at least, until you buy and settle into your new home. When things settle down, you should pay any cash left over off your mortgage as it's likely that the mortgage rate will be a lot higher than the rate you get on your savings. It's possible that the bank will stipulate that you put your savings against your new mortgage anyway.