The main use would seem to be if we are to have a very slow electrification program.
There's no point buying electrical until the power lines are in place.
There's no point having diesel after the lines are in place.
Normally, you would say to buy diesels and cascade them to areas where there is no electrification. That's not so easy, with the Cork Commuter service being the only significant operation outside Dublin suitable for Dublin's retired diesels. Even if massively expanded to double frequency and train capacity on current services it could take at most 15% of diesel railcars.
So in the absence of a big bang approach to electrification on the Dublin commuter network, hybrids become necessary.