According to Irish folklore, it was said that the cure for a hangover was to bury the ailing person up to the neck in moist river sand.
This could have the same effect as a cold shower. It’ll wake you up and get the blood pumping, but there’s not much medical evidence that it cures a headache and nausea. Really, this seems like another prank-turned-legend. “Up all night drinking poitín, is it? To the river!”