text: They are official road reference numbers, used by the Roads Service or TransportNI or whatever they call themselves these days. 
   
  They are often used in news reports regarding road closures ie http://www.coolfm.co.uk/news/local/road-closures-due-to-flooding/  and also in official reports from TransportNI to the local councils.  - https://www.infrastructure-ni.gov.uk/sites/default/files/publications/infrastructure/fermanagh-and-omagh-district-council-report-spring-2016.PDF
  
  U or Unclassified roads should be tagged highway=unclassified and are public roads maintained by the local authorities.  For more info see the wiki - http://wiki.openstreetmap.org/wiki/WikiProject_Northern_Ireland
  
  