Halifax are paying the fees (possibly up to a €1000 limit), if your LTV is low and you current rate is rubbish then it can be a great move.
The ECB rate is a rate that the European central bank lends at. Most banks get there money on the Euribor market (bank to bank lending) and this rate is normally higher than the ECB. But whatever rate they get their money at, the customer will pay more. I think AIB's stan var is ECB+1.25%, this 1.25% (close to it) is the way the banks make money, its there commission on the money.
Thats why they dont want people on trackers as the usual rate is ecb+1% or less, meaning they make less money or possibly none.