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.