The N81 road is a national secondary road in Ireland, from the M50 motorway to Tullow, County Carlow, north to south. The N81 continues past Tullow for another 8 km to terminate at the village of Closh, County Carlow, where it intersects the N80. The N81 is 76.784 km (47.711 mi) long (route map). The road is a dual carriageway between M50 motorway and west of Tallaght, known as the Tallaght Bypass or Blessington Road. It intersects with the M50 motorway at Junction 11. There are plans to extend the dual carriageway by 5 km (3.1 mi) to the urban boundary.

The N81 is the only major national road emanating from Dublin that is a national secondary rather than national primary road.

The official definition of the N81 from the Roads Act, 1993 (Declaration of National Roads) Order, 2012 states:

N81: Dublin — Closh Cross, County Carlow

Between its junction with M50 at Templeogue in the county of South Dublin and its junction with N80 at Closh Cross in the county of Carlow via Tallaght Bypass, Blessington Road, Jobstown, Gibbons, Corbally, Crooksling and Brittas in the county of South Dublin: Moanaspick, Tinode; Main Street at Blessington; and Burgage More in the county of Wicklow: Glebe East in the county of Kildare: Burgage Moyle and Russborough in the county of Wicklow; Bishopslane and Horsepasstown in the county of Kildare: Poulaphoca Bridge at the boundary between the county of Kildare and the county of Wicklow: Hollywood Lower, Hollywood Cross, Whitestown, Castleruddery; Mill Street and Edward Street at Baltinglass; and Holdenstown Lower in the county of Wicklow: Bough, Rathvilly, Kilmagarvoge; Dublin Road, Church Street, Market Square, Bridge Street and Abbey Street in the town of Tullow; and Castlegrace in the county of Carlow.