Might be worth adding the following from Wikipedia: https://en.wikipedia.org/wiki/Local_roads_in_Ireland#Local_road_numbering "The local primary roads are numbered from L1000 - L4999. Local secondary roads are numbered from L5000 - L8999. Local tertiary roads are numbered from L10001 - L89999, with the first four digits representing the local primary or secondary road to which it is connected."
There are a lot of roads in OSM over L5xxx but marked as tertiary currently. Personally I would be in favour of naming tertiary roads as Lxxx-1. - dafo43
Northern Ireland road grades
I have raised the following note with respect to Belfast: http://www.openstreetmap.org/note/611462 and have been advised that highways are being (apparently) implemented across Ireland North and South, and that any road above tertiary level is tagged according to it's road grade (A/B/C).
I have absolutely no difficulty with that and think it's good that we look towards an all-Island focus (I'm not going to get political!), but it does present some issues in that as a result we have can have narrow single lane carriageways in the city centre that are rendered in the same way as near-to-motorway quality separated dual carriageways.
I think the simplest solution would be implementing more 'primary' rather than 'trunk' roads in Belfast and other NI towns, though as I can see that these aren't much in use in the Republic I wouldn't want to create difficulties there?