Several pages into Hades, the funeral carriages cross "The grand canal" at Victoria Bridge. Later in the chapter, they pass over the "Crossguns bridge: the royal canal." These two canals connect Dublin to the west of Ireland by way of the River Shannon.
The Grand Canal and its northern twin the Royal Canal were constructed in the second half of the 18th century and the first two decades of the 19th, to connect Dublin to the Shannon in central Ireland. Their long curving arms encircle central Dublin, ending near the Liffey in the eastern edge of the city. Each canal is 80-90 miles long (130-150 kilometers), with many locks. As he crosses the Crossguns bridge, Bloom thinks of "Dropping down lock by lock to Dublin. With turf from the midland bogs."
This bridge—officially named Westmoreland Bridge when it was built, but the name didn't stick—carries Phibsborough Road over the Royal Canal, leaving metropolitan Dublin for the northern suburbs in an area long known as Crossguns. Underneath the bridge is a lock allowing canal barges to be lowered from the landward (western) level to the lower level that is maintained to the east. As the funeral carriages pass over the bridge, their occupants can see that the lock is currently in use: "Water rushed roaring through the sluices. A man stood on his dropping barge, between clamps of turf. On the towpath by the lock a slacktethered horse." Bloom thinks that the canal goes through Mullingar, and he could visit his daughter by walking or cycling along it.
The canals still served an economic purpose in 1904, but the advent of motor vehicles announced their coming obsolescence. Barge traffic was discontinued in the mid-20th century.
Bloom thinks also of the foul condition of the water in the canals: "slime, mudchoked bottles, carrion dogs." In Ithaca he thinks of the dire "possibility of recourse being had to the impotable water of the Grand and Royal canals as in 1893," Dublin currently being in the grip of a drought.