First of all don't worry too much about the tracker. It's worth about 1% a year, reducing quickly, so maybe it's worth €3,000 to you. Take it into consideration, but the other issues are much bigger.
So you need to decide if you should withdraw the pension from the previous employer. It probably does not make sense. Leave the money there growing tax-free. That is a bigger, longer term decision than trying to keep the tracker.
Ulster Bank may allow you to take over the tracker. Ask them. If your income is over €30k, they probably will. If they don't, you can take out a mortgage with them at 2.25%. So it's a pity to lose the tracker, but not a deal breaker.
Has your ex agreed to the buyout? Does he need the cash now himself? If UB refuses to allow you to move the tracker and you can't refinance it anywhere else, then you could do a deal with your ex to transfer ownership but keep the mortgage. It's messy and better to have a clean break.
Brendan