The Grand Canal begins in Grand Canal Dock at the River Liffey, and continues through to the River Shannon. It passes through Ringsend and traverses the southside, delineating the northern extremities of Ballsbridge, Ranelagh, Rathmines, Harolds Cross and Crumlin. This section, known as the Circular Line, has seven locks. The path of the original main line, which serviced the Grand Canal Harbour, the City Basin (reservoir) and Guinness brewery, can be seen at Inchicore. Most of the route of this line now runs alongside the Red Luas Line.
From Suir Road Bridge, the lock numbering starts again at 1 as the canal heads west through the suburbs of Dublin West and into Kildare. At Sallins the Naas/Corbally branch diverts southwards, while the Grand Canal continues west passing Caragh, Prosperous and Robertstown, its highest point. Just outside Sallins, the canal passes over the River Liffey at the Leinster Aqueduct. Just east of Robertstown is the location where the Blackwood Feeder used to join the canal, whilst just to the west can be found the busiest junction on the canal where the Old Barrow Line, Milltown Feeder and the entrances to the Athy & Barrow Navigation meet. Further west, the canal passes Edenderry, Tullamore, Rahan and Pollagh before it reaches the Shannon at Shannon Harbour in County Offaly. In total the main line of the canal is 131 kilometres (81 mi) with 43 locks, five of which are double locks.