The M4-M6 motorway links the N4 and N6 National Primary Routes west of Kinnegad to the existing M4 motorway at Kilcock.

The scheme begins at the end of the existing M4 motorway at Kilcock, from where it passes south of the R148. A full grade separated interchange is provided at Kilcock connecting the Naas road to the Trim road.

The route continues in a westerly direction towards Cappagh Hill, passing to the south of the Cappagh GAA pitch, crossing the Ballycorron River and continuing towards the regional road between Edenderry /Johnstown Bridge and Enfield, passing to the north of Johnstown House. A full grade separated interchange is provided to serve Enfield and Johnstown Bridge along with the Trim and Edenderry Roads and links with the Enfield relief road.

The route continues passing south of the railway and the Royal Canal, crossing the rivers Blackwater, Togher, Aleckafin, Glash and Boyne. It crosses the regional road to Edenderry and the Kinnegad River passing south of Kinnegad.

The route provides free flow connections to the N6 (Galway) and N4 (Sligo) to the west of Kinnegad. At Kinnegad grade separated interchanges are provided on the N4 and N6 links. Over and underpasses are provided to cater with the local traffic through the motorway junctions.