An interesting question.
If you have a mortgage with UB and they cease doing business, they can't call in the mortgage to be paid early. In theory, your terms and conditions are not changed. However, as it's a SVR mortgage, they can up the rate at their own discretion and there is nothing you can do about it.
While other banks can up their SVR too, they keep them competitive as they wish to attract new business.
If UB closes, they could sell on their mortgage book and you could lose protection of the Central Bank.
So, taking all these factors into consideration, you should see if there is an option to move to one of the active banks i.e. AIB, BoI or KBC. ptsb is probably ok as well, but there is uncertainty over their future as well.