The N51 runs from junction 10 of the M1 at Mell, west of Drogheda in County Louth to the N52 at Delvin in County Westmeath by way of Slane, Navan, and Athboy in County Meath. It intersects the M3 at Junction 9 west of Navan. The section from the M3 to Navan is D2, with the rest of the route being relatively poor quality S2 or urban S2/S4.
Drogheda
The N51 was diverted north of Drogheda with the construction of the North Cross Route. The previous route into the town had been via the now unclassified Slane Road and then the R168 Trinity Street. The North Cross Route saw it end on the N1 at Rosehall Roundabout. Following the opening of the M1, the N1 was downgraded to R132 and was re-signed. Subsequently, the N51 has also been downgraded between M1 J10 and Rosehall Roundabout, becoming R166 and R168. While this change has not been reflected in the signage, the routes are listed in SI/2012/54.
