Joe F,

I did some searching, and didn't find any other mentions of the Catholic/Protestant interpretation. I did find

THE HISTORY OF "THE FROG'S COURTSHIP": A Study of Canadian Variants

which you might find interesting.

The song is discussed in the book Hidden Fermanagh by Cyril Maguire. From a review of the book:

The reference to Seán Corcoran's collection appears in a discussion of a song he collected from Annie McKenzie, The Frog's Wedding, where, unfortunately, while admitting the possibility of 'allegorical interpretations', Cyril gives credence to the view that 'it is simply a children's song made up and sung for enjoyment' (p.61) espoused by John Moulden (sadly, misspelt here as 'Molden').

Now, forgive me if I'm wrong, but songs deliberately aimed at children do not usually include verses such as this, which seems not to require any allegorical interpretation whatsoever:

"Arragh, Missie Mouse, will you wed?"
Fol-lie, linkum-laddy,
"Will you come into me bed?"
Tidey-ann, tidey ann, diderum-diedum-dandy.

Anyone with any knowledge of ranarian sexual behaviour knows full well that male frogs are incessantly promiscuous and, as a result of their myopia, are not always accurate in terms of the direction of their Priapic intentions. Indeed, this means that when froggie feels the urge to merge, he'll jump on anything that bears the slightest resemblance to the generative obligations of his species (including many a poor mouse!). And what are we to make of the line 'Frog rode up to Mouse's hole'?