In the UK, we often hear of roads in Northern Ireland being called "The X Road" in the news. This isn't common usage in Great Britain. I can think of five reasons why this may be common usage, but haven't been able to work out which one dominates as each seems to maybe apply sometimes:
- The usage refers to a general area by metonymy;
- The usage is a differential linguistic marker of community membership within NI;
- The usage is uncomplicatedly common throughout the island or within NI;
- The roads change name a number of times throughout their length, and this phrasing is used to denote the whole length in contrast to the named parts;
- The usage is no more common than in the UK / the sample is incomplete / this is selective memory, etc.