Motorways in the Republic of Ireland
In Ireland, the highest category of road is a motorway (mótarbhealach, plural: mótarbhealaí), indicated by the prefix M followed by a one- or two-digit number (the number of the national route of which each motorway forms a part). The motorway network consists entirely of motorway-grade dual carriageways and is largely focused upon Dublin.
Source: Wikipedia — Motorways in the Republic of Ireland (CC BY-SA 4.0)