What's the problem? The three trains in question all go non-stop to Skerries after leaving Connolly. Surely it's OK for some trains to go non-stop and some to call at each station? Limited-stop services are important in driving demand to more distant locations, as it gives (or should give!) a more favourable journey time...especially on slow-to-stop-and-start-again DMUs
I don't know the recent population increases around Rush and Lusk too well but it seems that just adding the population of two towns and assuming that should mean better service is a little imprecise when it comes to setting timetables. Passenger demand, not to mention the catchment area of the station (i.e. 5000 people in a town that extends a square mile from a station is different to 5000 people spread over ten square miles!) would be an issue here. The fact that the station is called "Rush and Lusk" and the census areas of the towns of Rush and Lusk add to a certain number doesn't, in and of itself, mean that there is enough demand to justify the change...IÉ says that Rush town is 2 miles from the station, so it's optimistic naming anyway...
On the other hand, the 1713 does stop at all stations on Saturdays so maybe that one could try an all-stop service on weekdays and see what impact it has? Surely worth a try?