Some banks don't access ECB funding though, or have a blended funding rate, so shouldn't any tracker be at "cost of funds plus X"?
I certainly think so - at least in in theory!
The fact that banks thought that the ECB refi rate would always be a reasonable proxy for their cost of funds was a significant contributor to our banking meltdown.
On the continent, it is more common to use 12-month EURIBOR as the reference rate for determining floating loan rates but this can suffer from the same problem as using a ECB policy rate, particularly for smaller, less highly capitalised, banks.
In the US, I gather it is quite common (although by no means universally the case) to use a bank's cost of funds (COF) as the reference rate for adjustable rate mortgages (ARMs). Larger banks will sometimes use their their own internally calculated COF but this obviously suffers from a lack of transparency. However, most banks set their ARM rates by reference to a regional COF index that is calculated by a third party.
We could certainly do something similar in Ireland. At least in theory.
Incidentally, I like the idea of a variable rate mortgage with an interest rate cap - either for an agreed period or for the full term of the loan. Effectively, a borrower gets the flexibility/potential upside of a variable rate with some of the advantages of a fixed-rate loan. At current long term swap rates, I would have thought that lenders should be in a position to offer such a hybrid product without imposing a material premium for the downside protection to the borrower.