The R378 road is a regional road in County Mayo in Ireland. It runs along the south shore of Clew Bay and connects the R335 in Louisburgh to Roonagh Quay, away (<span class="plainlinks">map of the route</span>).
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: