Ulster Bank has committed to making the rates for new customers available to existing customers. They also allow existing customers avail of lower LTV rates if their LTV falls into a new category.
AIB has made no commitment but it has been their practice to pass on rate cuts equally to new and existing customers. They don't allow you avail of the lower LTV rate if your LTV falls.
As they are the only two lenders to be treating their customers fairly, I would not take out a loan with any of the other lenders who does not do this.
It's all very well to say "Take out a loan with KBC and switch if they don't pass on rate cuts". Most people don't get around to switching. And some will not be able to switch e.g. if they change job or if their income falls. Look for a lender who treats you reasonably well, not a lender who waves sweeties in your face, and then screws you as soon as you are an existing customer.
ptsb now has the same headline rates for new and existing customers, but they compete for new business by offering 2% cash back and a 0.5% discount in the first year. This allows them keep the headline rates high.