Our national road speed limit signage does not make sense

I paid up and resolved to pay more attention to speed limits and cameras on the 15-mile ordeal of the coast road R336 whose gauntlet I must occasionally run to get to Galway (access to public transport is confined to a single daily bus).
Despite my extra care, within three weeks, an extra three points and €80 were added to my record. It demanded closer investigation before I would resignedly pay up again.