If another LoCo came in, Ryanair would dump capacity on their routes. Like happened with Go or EasyJet.
So, any airline in question would have to come in with deep pockets and also with the intention of basing at least 4 or 5 aircraft in Dublin.
Ryanair have been strangely tolerant of Wizzair on Ireland-Poland, but there seems to be significant crossover in shareholders between the two companies.