It's about customer inertia, tomorrow Rabo could cut the rate on that instant access product to e.g. 0.75% and about half the money will stay there because Irish people are lazy about moving their money around - why else are there still so many people with savings in AIB/BoI/Anglo.
What Rabo could do then is offer a new instant access acc e.g. paying 2.25% and some customers will move their money out of the old one into the new one. In the meantime, the fixed term rates cannot be varied.
In practice, what will happen is that as short-term market rates increase, Rabo (and most other banks) most likely will not pass through those increases to the instant access account and will instead make new offers on e.g. fixed term deposits. So don't be surprised if the Rabo instant access is still paying 2% in a year's time, when short-term overnight rates have increased from their current levels of approx 0.70%
http://www.euribor-ebf.eu/euribor-eonia-org/about-eonia.html. So at the moment, Rabo are paying up 1.30% over the overnight rate, and overnight rates are forecast to hit 1.25% by approx september of this year. It is unlikely that Rabo will increase their rates from current 2% to e.g. 2.50% in september - but I could be wrong!
One other point - you will notice I mentioned nothing about ECB, as realistically Rabo's funding costs are nothing to do with ECB, they do not pay ECB (1.0%) for funds on the Interbank market. As a AAA rated bank they would be paying below the 0.70% overnight rate mentioned above. A typical Irish lame duck bank would be lucky to get cash interbank overnight at 1.50%.