I think the beer is to kill the taste of the food.

Did you know there's no corn in corned beef?
"Corned beef so called for the 'corns' or grains of salt with which it is preserved; from verb corn "to salt" (1560s)."

So today, enjoy your salted meat, your warm stout, your boiled cabbage and potatoes, your soda bread, and wonder why your ancestors left "the old country."