Why?
Because they are wasting millions trying to develop a bespoke system, that will restrict competition, given the three balls re restricting who can join the system, and initially only be available to retail customers.
SEPA Instant is already available, and the defacto European stand system, with well over a hundred European banks and financial services firms already using it. It could be delivered to the customers of the three Irish Banks faster, cheaper, and offer more capability than initially proposed by the Irish version.
That's why
Except it can't be. Introducing sepa instant into the Irish banks that have decades of built up payments infrastructure is not straight forward.
Traditional banks have operated in 'batch' mode for decades. Moving away from that to real time processing is anything but simple. Revolut and other fin tech companies don't have the issue of legacy systems.
SEPA as a concept took nearly 15 years to be fully implemented across Europe and that was a batch process that banks understood.
All that is also ignoring the fact that SEPA instant is simply a foundation. Most banks in Europe even those who have SEPA instant are looking at solutions like this app. And that's because as good as SEPA instant is, the really powerful stuff is through open banking and account to account payments.
As for SEPA instant being the European standard, it's not. There is a reason why the ECB has not mandated banks to adopt it. If as you say it is so easy, cheap to install and the answer to everything, the ECB would have given every bank 5 years to implement it. Even now, they are not mandating banks to offer it. Only just over 50% of European banks offer SEPA instant. When it comes to payment, that is not a standard. SEPA is a standard. SWIFT is a standard. SEPA instant isn't.
As for the APP itself, I know very little about what they are doing but as far as I know it will be available for any institution to join. Banks often cooperate to create market solutions. SWIFT itself is owned by its members. There is nothing uncompetitive about it.