Since June 2022, ECB has increased rates by 4.5%, while AIB for example have increased their 5 year Green (50-80% LTV) Fixed rate by 1.6% to a position where their rate is now below the ECB rate. Had AIB increased by the same amount as ECB, the fixed rate on offer would be 6.65%!
So it seems they're funding new mortgage business from excess customer deposits. Should customer deposits start to drop as the cost of living/inflation eats into savings, I suspect the Irish banks (or at least AIB) wont be as quick to reduce their fixed rates as the ECB reduce theirs.
AIB customer deposits grew every quarter from Dec 2011, until June 2023 when deposits actually fell by 0.9%.