The government legislation that defines the R378, the Roads Act 1993 (Classification of Regional Roads) Order 2012 (Statutory Instrument 54 of 2012), provides the following official description:
Louisburgh — Roonagh Pier, County Mayo
Between its junction with R335 at Bridge Street Louisburgh and its terminal point at Roonagh Quay via Accony all in the county of Mayo.[1]