The government legislation that defines the R318, the Roads Act 1993 (Classification of Regional Roads) Order 2012 (Statutory Instrument 54 of 2012), provides the following official description:
Foxford — Pontoon, County Mayo
Between its junction with N26 at Bridge Street Foxford and its junction with R310 at Cuingbeg via Corlummin all in the county of Mayo.[1]