The 'catch' is that you must stay with them for five years. if you leave them within 5 years, you must repay them the legal fee they gave you in th beginning to pay for the switch.
The other catch is that you may only pay a minimum excess of €1,000 off your mortgage at any time. EBS for example, calculate the interest rate daily, so if you knock an extra 20€ a week off the capital of your mortage with EBS, your capital will reduce more quickly, but so will the overall interest you pay on the mortgage. Therefore, the EBS method may actually save you more money (cheaper mortgage) than BOSI. Test this on amortisation tables- there are plenty of them online.
The tracker rate will be 1% only over the ECB rate for the lifetime of the mortgage (for LTV of 75% or less)