The official description of the R120 from the Roads Act 1993 (Classification of Regional Roads) Order 2012[1] reads:
R120: Lucan - Rathcoole - Corbally, County Dublin
Between its junction with R835 at Lucan Road in the town of Lucan and its junction with N7 at Rathcoole via Fitzmaurice Road, Adamstown Road and Lock Road in the town of Lucan; Twelfth Lock, Milltown; Peamount Cross, Newcastle; Rathcreedan, College Lane and Fitzmaurice Road all in the county of South Dublin.