Hi Paulie
It's a very good question and very hard to answer. Here are my current thoughts on it.
Ulster Bank treats its existing customers exactly the same as new customers, so if the rate difference is not too big, then I would go variable with Ulster Bank.
AIB and ptsb pass on rate reductions to existing customers as well. However, if you start with a 90% LTV mortgage, and you later reduce the LTV to 80% or 60%, they do not allow you to avail of the lower rate. Ulster Bank does allow you avail of the lower rate.
(permanent tsb does not offer existing customers fixed rates, but that should not be a factor in your decision, as it will be remedied shortly.)
Bank of Ireland is tricky. They have not cut their Standard Variable Rate. Existing customers have no right to move to a lower LTV rate - however, if you threaten to leave, then you will be able to get a lower rate. To make matters more complicated, they give 2% cash back to new customers but you must stay with them for 5 years. And they have competitive enough fixed rates if you want to fix. ( My personal view is that fixing at rates around 1.5% to 2% above the Eurozone average is not a good idea, and this is debated elsewhere.)
KBC offers new customers rates which are lower than other lenders, but they screw existing customers. Avoid.