Well (going way out on a speculative limb here) ...
from a purely business perspective, assuming RE staff are paid less than their EI counterparts, and perhaps are more productive (if that can be measured in any reliable way), it could make sense for EI to take a minority stake in RE and then wetlease RE to operate certain shorter haul flights with the ATR's in EI livery that would otherwise be overkill to deploy a 320 onto.
It would also provide for a platform to test new routes using ATR's out of Gatwick before deploying 320's (or similar) onto them.
In the current tough times for Ireland, it could be a good arrangement for both airlines if EI can survive the unions throwing their toys out of the pram. Just a thought.
JAS