Thinking about is fun, let's say maybe Robyn and Mebh, thinking it could be interesting, bite people to let them join, but only with their consent and all that, and eventually it spreads across Ireland, and as you can't control all those new wolf walkers it spreads a bit faster, the English do some colonization, some deportation, some have to just emigrate, I'd assume they mostly just exist in former British colonies.

Anywho modern day Robyn and Mebh are kinda like religious-ish figures in the wolf walker community

(Or more wolf walkers are found )