Here's what seems like a major contributor:

https://nicoleevelina.com/2013/04/22/poets-most-powerful-celtic-satirists/

With these sources:

Secrets of the Druids by John Matthews
Magic of the Celtic Otherworld: Irish History, Lore and Rituals by Steve Blamires
Celtic Myth and Religion: A Study of Traditional Belief by Sharon Paice Macleod
http://loki.stockton.edu/~kinsellt/litresources/celts/review/supernatural.html

Some the things in the post seem to be from the book "Celtic Smiths and Satirists: Partners in Sorcery" by M. Randolph

Although I think the "they weren't allow to marry" was more that they became hated for abusing their power, so it was more likely a "they shouldn't be allowed to be happy" type of deal. But I didn't read enough to check that out fully.

Yeah, none of that explanation sounds quite right.