It is quite clear that FR would not offer "a huge amount of feeder traffic", why? Because EI is a larger airline than FR out of DUB (even if only just). If they were to "merge" FR would add few truly new city pairs, or catchment areas to the EI network. A true comparison of the new cities added is difficult, because FR often serve similar regions out of different airports. One could probably argue that Leeds, Liverpool and Manchester all cover a similar catchment area, the airports certainly would.
Also, the assumption is made that FR would interline with EI and I dont think that is a safe assumption. Also remember that a lot of FR routes are seasonal and/or only flown a few times a week, which may, or may not make for convenient connections, if these were even allowed in the first place.