|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 (former T3) 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 railway terminus. From here, the route continues westward along the Liffey valley, although often a little removed from the river itself, through the districts of Kilmainham amd Ballyfermot, before reaching the M50 (junction 7) at Palmerstown, 9 km from O'Connell Bridge.
Our 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.