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:

In her 1999 publication South County Scrapbook, Mary McNally of Tallaght Historical Society noted that: "There was no road between the Embankment and Crooksling (route of the present-day N81) until about 1820 when British Army Engineers made the present road. Before 1790, the travellers going from Dublin to Blessington, Carlow and Wexford turned left at Kiltalown Cottages and climbed the steep road over Mount Seskin joining the present road (N81) at Brittas."

The section of the N81 south of Tallaght which circles Verschoyle's Hill (386m), was once the site of highwaymen, which academic Francis Elrington Ball wrote of in 1904:

The opening of the eighteenth century found this district still very wild and uncivilized, and the old coach road to Blessington which passed over Tallaght Hill, was a great resort of highwaymen. At the foot of this hill, on the Blessington side, there stood an inn, called the Red Cow, which, in the month of December, 1717, was the scene of a sanguinary encounter between a party of rapparees, who seem to have had the surrounding country at their mercy, and the forces of the Crown.

In his 1877 book The History and Antiquities of Tallaght in the County of Dublin (reprinted in 1899), William Domville Handcock made note of a pillar stone, or gallán, which "stands opposite Mount Seskin in the loop formed by the new Tallaght road going round Tallaght Hill from Kiltalown to Brittas (the modern N81 road) and the old steep road between those places (the L7377 Mount Seskin road)".

The N81 route was previously known as the T42 (trunk road), trunk roads being a classification used in Ireland between 1926 and 1977.