If there is a problem at the level crossing there can be traffic paralysis round the area of the schools. DM joins the road to castleknock village out beyond the carpenterstown area / bracken / collegewood so it should be a lot more protected from this. It is an 18 minutes walk from the far entrance to DM to the train station, going the "giant three quarter circle route" ignoring any pedestrian links into the beeches / the school area or even onto the porterstown road itself.Parts of the future development of HP are further away again from the station.
From experience, the build up of traffic getting into Castleknock Village from that direction can be crazy - I can't see it being any better than the traffic around the Carpenter. DM will be closer to the train station than part of HP perhaps - I guess it just feels quite disconnected from the neighbourhood than HP, given that lack of road access