Ireland, both north and south of the border, has an extensive network of roads. Northern Ireland has had motorways since the 1960s and well developed primary routes. With the advent of the Celtic Tiger and European Union funding, most national roads in the Republic continue to be upgraded. In the 1990s, the Republic went from having only a few short stretches of motorway in the country to partly expanding the motorways on most major routes. This is part of a National Development Plan. Road construction in Northern Ireland (where the road infrastructure was traditionally of a far higher standard than the Republic) has proceeded at a slower pace in recent years (To the extent that some even claim that it has now been overtaken by the Republic), although a number of important bypasses and upgrades to dual carriageway have recently been completed or are about to begin.

Signposts in Northern Ireland denote distances in miles, while all signposts placed in the Republic since the 1970s use kilometres. Currently miles per hour speed limits are used north of the border. Those in the Republic use kilometres per hour (km/h) introduced on 20 January, 2005. This involved the provision of 58,000 new metric speed limit signs, replacing 35,000 imperial signs and adding 23,000 new signs. The Republic's road signs are bilingual, using both of the state's official languages, Irish and English.

Roads in the Republic of Ireland
The Republic's motorway network is focused on Dublin, and is currently being extended to other major cities as part of the National Development Plan. Dublin has also been the focus of other major projects, such as the East-Link and West-Link toll-bridges, as well as the Dublin Port Tunnel.

Motorways
In the Republic of Ireland, the most major roads are motorways; indicated by the prefix "M" followed by one or two digits. The motorway network is focused on Dublin. The first motorway section in the state was the M7 Naas by-pass, which opened in 1983. As of 2005, all motorways in Ireland are part of, or form, national primary roads. These routes are numbered in series, using numbers from 1 to 33 (and separately from the series - 50). Apart from the motorways, these routes carry an "N" prefix. At the end of 2004, there were 191.7 km of motorway in the Republic and 285.5 km of dual carriageway. This was extended, by the end of 2005, to 246.6 km of motorway and 297 km of dual-carriageway. In addition, 17.4 km of dual-carriageway has been reconstructed, specifically 15 km between Naas and Dublin and 2.4 km on the Cork Southern Ring road. Most of the new dual carriageways being built in Ireland are built to motorway standards with full grade separation and controlled access, however to accommodate slow moving vehicles e.g. tractors, the motorway designation [M] is not used. It was revealed in August 2006, that the National Roads Authority (NRA) were considering reclassifying around 500 kilometres of high-quality dual carriageway to motorway, dramatically increasing the motorway network in Ireland. On 5 March 2007, a Bill was published, the Roads Bill 2007, which if passed by the Oireachtas will make this reclassification possible by Ministerial order.

Route number inheritance
In the Republic of Ireland, motorways use the route number of the national road they form part (or possibly in the future, all) of, albeit with the M prefix rather than N. In most cases, the motorway has been built as a by-pass of a road previously forming the national road (e.g. M7 by-passing roads previously forming the N7) - the by-passed roads are reclassified as regional roads, although updated signposting may not be provided for some time, and adherence to signage colour conventions is lax. Regional roads have black-on-white directional signage, national roads use white-on-green (with the route numbers in amber). Motorways in the Republic of Ireland have white-on-blue signage. Destinations reached by other classes of routes should be listed on a correctly coloured "patch", except on motorways, where all signs should be blue.

The M50, an entirely "new" (1989) national road, is an exception to the normal inheritance process - as it does not replace a road previously carrying an "N" number. The M50 was nevertheless legislated as the "N50" route (despite having no non-motorway sections, other than a very short section at Tallaght that has been subsequently subsumed) to record this national road number in the statute books. It was thereafter designated M50 due to the route being entirely motorway standard. The route also breaks the sequential numbering scheme, but 50 was deemed an easily recognisable number. It is referred to by the NRA in its "National Route Lengths 31/12/2004" publication as the N50, as the publication does not distinguish motorways from national primary roads. Instead, it classifies motorways where present as the national primary road under an N designation, for example its states there is 14.21 km of the N1 in County Meath while also stating there is 14.21 km of motorway in Meath.

Motorway sign in Ireland

The NRA have designated the M1, which indeed is 14.21 km in length in County Meath, as the N1 in the document (the road which was previously part of the N1 through Meath is only approximately 8.5 km long). As of 2005 N34 is the next unused national primary road designation.

An interesting anomaly occurred on the 31st May 2004 in Limerick, when the N7 Parkway scheme and the N7 Limerick Southern Ring Road which is parallel to and bypasses the N7 Parkway were opened simultaneously by the Minister of Transport. The N7 Parkway scheme was labeled as such to get National road funding and within minutes of opening was immediately redesignated a regional road, so as the Minister could open the N7 Limerick Southern Ring Road. NRA.