|Location Map ( geo)|
|Distance:||70.7 km (43.9 miles)|
|Meets:||R105, R138, R108, R803, R804, R805, R109, R111, R839, R833, R112, M50, N4, R403, R419, R404, R449, R157, R406, R408, R407, R125, R158, M4, R402, R159, R160, R401, R446, R161|
|Former Number(s):||N4, T3|
The R148 follows the original, pre-motorway, route of the N4 Dublin–Sligo road from the centre of Dublin as far as junction 13 of the present M4/N4 at McNead's Bridge in the townland of Heathstown between Kinnegad and Mullingar, County Westmeath.
Starting on Aston Quay, at the south-west corner of O'Connell Bridge, it first runs west along the south bank quays of the Liffey, turning left into St John's Road West at the Frank Sherwin Bridge to pass along the southern side of the Heuston station, crossing the railway on John's Bridge.
From here, the route continues westward along the Liffey Valley, although often a little removed from the river itself. St John's Road West was originally the N7, while the Chapelizod Bypass opened in 1987. At the time this opened, the junction near Kilmainham would have been one of the first times traffic would have needed to stop.
From Palmerstown Interchange, the route is now subsumed into the first 6.5 km of the N4, before that road becomes the M4 at junction 5 at the western end of the Lucan Bypass, and the R148 sets off once more, running for some 55 km through Leixlip, Maynooth, Kilcock, and Kinnegad, finally rejoining the N4 where that route's motorway section ends, at the above-mentioned junction 13.