Two possibilities:
It's a generic christian tale about a saint driving out evil
It's one of many examples of tales about gods (particularly Lugh) being retconned with Patrick as the protagonist. There's quite a lot of stories about Lugh/Lughos/Lleu or a christian stand-in killing snakes (probably cirrostratus clouds) to release the rains. He's also supposed to have done other things like summoned demons to murder unbelievers and many other very obviously non-christian acts.
oh so it might be a recycled story like so many been
That would be my guess. There's a lot of stories about Patrick and Columbanus that have zero christian elements, only a christian veneer, or even patently non-christian elements.
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