The R639 runs parallel to the M8 from its junction with the N77 in Durrow in County Laois to the N8 on the Lower Glanmire Road 1.5 km west of the Dunkettle Interchange on the outskirts of Cork City.

North to south, it passes through Durrow, Cullahill, Johnstown, Urlingford, Littleton, Horse and Jockey, Cashel, and New Inn; and from there it runs south-west around Cahir and on through Skeheenarinky before entering County Limerick and passing through the village of Kilbeheny before veering west of Mitchelstown and proceeding south along Kilworth Mountain and through Fermoy, Rathcormack, Watergrasshill, Sallybrook and Glanmire, terminating at the N8 Lower Glanmire Road.

Much of the R639/N8 route was built to connect the midlands to southern Tipperary and north County Cork as part of the Irish turnpike road-building drive of the mid-18th century.

[2] However, some sections are considerably older, such as the segment between Cork and Fermoy, and that between Cashel and Cahir; the construction of these particular stretches cannot as yet be dated, though they were in place prior to 1714.

The next major incident in the modern history of the R639 occurred in 1991, when a new 4 km single-carriageway bypass to the south-west of Cahir in County Tipperary opened to traffic.

For many years regarded as a serious bottleneck on the N8, a wide median 7 km dual carriageway bypass was constructed and opened to traffic in September 2003 as the N8, with the former road through Watergrasshill becoming the R639.

In October 2007 the section of N8 between Cashel and Cahir (the same road mapped by Herman Moll in 1714) was bypassed by an early-opening segment of the M8 motorway, and became the most recent addition to the R639 route.