This shanty was popular with the Brits when praising the virtues of that noble hero Santy Anna what went and fought them insurrectionist colonial Yankees.
The true(er) version was popular with the heroic upholders of freedom (them same Yankees) as it told the virtues of Gen Z Taylor. The *Molly-Del-Ray* was sailor jargon, and it was sailors what sung it!!!That is also why some versions include Boney, as sailors were always well educated scholastic sorts, and liked to include references to any old sod that did well in OTHER shanties :-)
You can sing the chorus as *Plains of Mexico*, *Bay of Mexico*, *Off to Californi-o*, or any other damn *i-o* and still be correct.
You may include Ranzo, his son, his father, Stormalong John and HIS son/father, Boney, Wellington, the Tsar of all the Russias AND his uncle. If the job at hand was long enough (pumping out the bilges of a tea-strainer, for instance) then sooner or later the whole world would go by, a bit like Grand Central Station.
I prefer the *heroic Santy Anna* version myself, but I never sing it without explaining that the truth was totally different, and was only sung this way by us Brits, 'cos we wuz thrashed by them Yankees a couple of years earlier, and wanted to get even.