I must admit that I was unaware that the Grand Canal forked [had a spur] at Suir Road Bridge.
The Main Line of the Grand Canal now runs from Suir Road Bridge, in Dublin, to the River Shannon. The Circular Line still runs through the south side of Dublin to Grand Canal Docks at Ringsend and then connects to the River Liffey.
At Suir Road Bridge the Grand Canal goes under the bridge. Immediately afterwards it makes a sharp right turn, goes under the (much older) Griffith Bridge and then travels through Dolphin’s Barn, Portobello and Dublin 4 to the Grand Canal Docks and eventually the River Liffey. This section is actually the Circular Line on which the locks have their own separate numbering system [this has confused me in the past]. The First Lock is at Suir Road and the 7th is at Portobello. Until now I could not understand why the Lock at Suir Road was known as the First Lock.
The Circular Line, the more recent section, was constructed between 1790 and 1796. The Main Line originally went straight on, to the Grand Canal Harbour at James’s Street. My understanding is that this section of the canal was closed in 1974 and then, between 2001 and 2004, much of it became part of the route of the Red Line LUAS tram system.
Between the Suir Road Bridge and Griffith Bridge is Anne [Ann] Devlin Bridge, which carries the LUAS line.