So, hypothetically, and I say that because I have players who read this forum, let's say that there was a covenant in Hibernia. And the SG for that covenant was looking through the oh-so-useful Antagonists book, and said, "Oh! The Waimie! That's a great monster. I could totally use that! And they did all the work for me, calculating out the spawn at various power levels! That's incredibly handy!"
And then we remembered that There Are No Snakes In Ireland. Curse you, Saint Patrick!
How would you alter or re-skin the Waimie for Hibernia? I have some ideas of my own, but I know the writers of Contested Isle are here, and their knowledge of Irish folklore might suggest some mythical beasts that can replace a horde of giant snakes. And everyone is, of course, very creative.
PS: Zero points for saying, 'They're not snakes, they're Worms/Dragons/Serpents.'
Make it something other than a snake, but is otherwise exactly the same. Maybe it's a rat, or a sheep or a dog or a tree or a leprechaun or a giant caterpillar (too much eating those faerie leaves) or butterfly that the caterpillar grows into. It could be a bear that is washed ashore. Or maybe some Infernalist (or Bonisagus testing things?) found a way to import a snake into Ireland or tried to transform something into a serpent and got a wamie... St Patrick might have banished the snakes way back when but that doesn't mean he keeps rebanishing them.
Maybe the Waimie (the big one) was the only one actually banished. But it doesn't apply to her children who were born much later. Don't have to change anything then really.
If that's not enough, maybe a site devoted to St. Patrick has gone in to disuse and finally allowing Waimie and her brood out. It could have been blocking their entrance but has broken through lack of faith and reverence to it.
St Patrick banishes snakes and serpents so I'd change the description of the Waimie such that she's a worm (worm proper rather than just worm in name). If I remember rightly, when I ran the Waimie encounter I described the Waimie and her brood as more worm-like anyway.
For those who don't know the story of Saint Patrick, he basically invoked Divine power to banish all snakes from Ireland forever. They aren't trapped somewhere, they just can't return. It's one of the many bits of local color in Mythic Ireland. I can ignore it, but I much prefer to work with it.
My first instinct was to re skin the Waimie and it's brood as a non-serpent. It is important to remember that whatever new monster it is made into, it will be siring or spawning hundreds of offspring which grow to enormous size. So it's not just a singular animal. I'm currently considering spiders; they are also poisonous, which is one of the most potent weapons of the Waimie's spawn. If I change them all to, for example, boars, poison does not make as MUCH sense (though the giant boar Trch Trwyth had poisonous bristles).
I was also thinking spiders for the creepiness factor and for the scenario where the serpent wraps itself around the covenant buildings so there is no entrance or egress, you could instead do that with spider webs.
I like the idea of a beehive or hornet nest as well.
Nightjar: this little bird has a dark reputation. They are nicknamed "lich fowl" or "gabble ratchet" according to wikipedia. HP Lovecraft used them frequently in his novels as sign of impending doom. It comes from the fact that they usually only sing at dusk, some say, to call the soul of those who are about to die. In large number, it can invoke Hitchkock's The Birds memories. They larger kin don't need to be made of flesh, but maybe become more like large shadow, leaving a feeling of dread and dispair to those touch by it.
Toads are also a "nice" reskinning. Ireland is wet enough to have them abundantly, and they can exist is all size. I believe it would be quite scary to see a whole field covered with croaking toads. Quite defeaning (hard to focus to cast spell), hard to navigate (one toad does not do much damage, but several jumping on you can make you trip, then they start to accumulate over you... yuck... buried under toads), if you step on one, it is very slippery. And toads can be poisonous as well.
Frogs and toads are also absent in Ireland thanks to St Patrick.
(Factually, there is one species of toad in very restricted locations in Ireland. Frogs have since been introduced, some time in the 18th century (possibly))
Even if St P permanently banished the serpents, maybe most people don't know the whole story, that keeping them banished involves a certain forgotten relic of St P remaining in a certain place or condition. But something has happened. Or maybe there is a condition on the banishment forever, also mostly forgotten.
So maybe the banishment sent the snakes and other nasties to various regios, and the barriers are weakening allowing some through. Reinstating the bansishment might prevent a general catastrophe, but the serpents powerful enough to already make it through must be dealt with by hand, because St P keeps the banished stuff banished, but he'd have to come back to rebanish.
Side stories here.
Having a serpent in Hibernia is much more interesting and disturbing than a spider, because of the very "problem" cited. Weren't they banished? What's going on? Is the story true? Etc.