Im Living in the Drogheda area, and as far as i know from my limited knoledge of the town, the spur to the cement plat runs south out of the main line, which would mean no train going north could stop in it and then continue. This understanding only comes from driving over the only remaining bit of track near the cement factory on my way to the station every morning. I also think this is the case due to a map of drogheda in a local barbers that shows the spur turning north-east out of the line and then south
A Drogheda north station would have to be slap bang in the middle of the main line, but with the end of the viaduct stretching out of the town it would have to be built in relation to further development on the north side of the town.
Also i have thought recently that it would be a good idea to replace the current span of the viaduct with a stronger structure capable of holding two tracks. Surely modern steel structures or some sort of strong carbons should be able to take the weight, and its not like the stone pillars cant take the weight. Im not an engineer but it looks like only the spans hold the viaduct back.