I wouldn't be in a hurry to move anywhere, tbh.
Ulster Bank will continue "as normal", into next year. If all goes according to (their) plan, some business customers will then be migrated over to AIB, with the rest later going to PTSB.
Given the significant volume of SME customers impacted by the future closure of Ulster Bank, its likely that there'll be some sort of automated, or at least "user friendly" arrangements for their SME customers to move to AIB or PTSB.
There's suggestion that some Ulster Bank staff will move to both AIB, and PTSB, as part of those arrangements. PTSB have also been recruiting some new staff, and are believed to be working on some technology enhancements, in anticipation of having a lot more business customers next year. Why not wait and see how that's looking, before moving to them?
Worst case scenario, you'll get (I think) a minimum of 60 days notice, to move /close your accounts. Given Ulster Bank's commitment not to close any branches or accounts this year, that letter is a long way off, if it ever comes.