The current short haul network from Aer Lingus appears very strong, we see good loads and decent mix of high frequency business routes and peak demand lead leisure destinations which must keep yield up for much of the year but where they do struggle is in finding new markets.
We've seen modest growth in short haul in the shape of one or two new routes each summer accompanied by frequency increased but those new routes come at a cost of other routes. Long gone are the days when Aer Lingus announced 5 or more short haul routes every summer!
The airline is growing overall so I think it's time they looked at the short haul fleet again, the A320 is too big for new routes and the A319 is costly to operate which is why they ditched them after a few years so a smaller jet is needed, the CSeries is fantastic, the EJet is tried and tested, even the Sukhoi seems a contender these days with a dirt cheap list price.
Unfortunately for Aer Lingus, the other IAG carriers don't need a smaller jet so IAG would need a lot of convincing to purchase a sub fleet for Aer Lingus when there's little to no business case across other parts of the group.