The presumption in all of this is that EI will cede some regional UK routes to RE.
If I am not mistaken, outside London EI only serves EDI, GLA, MAN and BHX with a grand total of 8 daily flights from DUB and 2 from ORK. What may make sense is to upgrade the daily EI-flights DUB-GLA/EDI and ORK-MAN/BHX to a twice daily ATR operation and maybe have an off-peak flight to BHX operated by RE. But in total, that would not take up much more than the capacity of one ATR72.
Finally, the reduction in capacity on the routes will most likely lead to an increase in frequencies which will aid EI in attracting connecting customers.
Hmm, but connecting from the UK to where other than the US? DUB is not exactly in a useful geographic location as far as pax originating from the U.K. are concerned.