From independence in 1922 until sometime in the 1970s, Irish roads were numbered differently. National routes were known as Trunk (T) and Link (L) roads instead of N(ational) and R(egional) roads.

Some roads followed the same path, such as T1/N1 and T2/N2, but most deviated greatly. For example, the road from Dublin to Cork ran from Naas to Athy parallel to the modern N9, before following the N24 (Waterford-Limerick road) as far as Cahir where it followed the modern N8 as far as Cork. The whole western north-south axis (today's N17/N18/N20) was T11 and a journey from Dublin to Rosslare utilised the T7 before changing to T8 at Enniscorthy, unlike today's N11 which ends at Wexford. Also some routes were not assigned numbers denoting importance. The N5 was listed as T77, meaning it was only a route of secondary importance, and the N3 was T35. There was no trunk number assigned to much of today's N9.

Surprisingly, it is still common to find old cast-iron road signs in remote parts of the country, still showing the old T/L road numbering - though these days, signs of this type usually end up on the walls of traditional Irish pubs the world over. It may be that the replacement program which the councils appear to have been unable to bring to completion may be completed for them by publicans.
Map key
Black : T1-10
Grey : T11+