Once they put in a motorway between 2 locations the National or Primary road as they used to call it gets downgraded to a Regional or Secondary road again in old speak. So basically a National road is the direct link between 2 towns / villages and Regional are all the off shoots.
National road speed limits (100 km/h)
A speed limit of 100 kilometres per hour is in place on all national roads (including dual carriageways) throughout Ireland.
Regional and local speed limits (80 km/h)
A speed limit of 80 kilometres per hour is in place on all regional and local roads (sometimes referred to as non-national roads).
Motorway speed limits (120 km/h)
A speed limit of 120 kilometres per hour is in place on all motorways. You should note that learner drivers, vehicles under 50 cc, bicycles, pedestrians, animals and invalid carriages are not allowed on motorways in Ireland.