Are you referring to the roads with L1234 etc. They've popped up everywhere recently.
As for the numbering scheme maybe they are sequential, just depends which end of the road you're on (eg the N3 and N4 are beside each other in Dublin, but at the other end they'd be out of sequence)
Most of the roads near us have 4 digits but there are a few that jump to 5 digits - seems unusual or maybe they just recently officially upgraded it to a 'road'. Maybe they'll give it a road surface now.