hi hans,
as with most prison jargon the obvious explanation is meant for general consumption. however the esoteric version is much more human and indicative of what occupies the majority of a prisoners thinking. sex.
the 'auld triangle' is a coloquial term for the genitals. ie in britain 'meat and two veg' [vegitables]etc
and the 'royal cannal' is quite simply a womans vagina. ergo the 'banks of,,,' refers to the womans thighs.
which in the song intends to show that no matter what time of day or circumastance, the 'auld triangle' is reminding its owner that its still there. which echo's in the use of the steel triangle to signal morning and lights out. [pavlov might have had something to say along these lines!]

hope this helps.
walt, wales :-)