The road travels through the towns of Clara, Tullamore & Portarlington.

The R420 commences at a roundabout junction on the R446 road east of Moate, County Westmeath.

From here the road continues in a south east direction where it forms junction 6 on the M6 motorway which connects Dublin and Galway.

The route continues for a short distance before joining the R445 road, west of Monasterevin.

The section of road was downgraded in status with traffic (when approaching Tullamore from the south) now encouraged to take the N52 to Kilbeggan and join the M6 motorway to continue their journey.