When St. Patrick drove the snakes from Ireland, did Cernunnos go with them?

Question: What is the relationship between the Nem and Tuatha? Do they see each other as kin? As tribes who follow similar laws? Or just "its weird Lugh's getting worshiped over in France"?

Ah, a question for every Storyteller: what to do with those cultures who occasionally overlap onto one another. While the Celts were a wideflung ethnic group with several distinct civilizations and languages, we often tend to think of them as one big shamrock-colored soup, which can make differentiating them difficult.

In the specific case of the Nemetondevos and the Tuatha de Danann, I would assume that the Tuatha are occasional visitors to the continental Celts; they have a history of wandering around to neighboring tribes and sharing their stories, and indeed Scotland and Wales are full of them as well. Particularly famous Tuatha like Lugh and Ogma did see a lot of worship on the continent as Lugus and Ogmios, which suggests that the two pantheons don't mind tolerating one another, at least, and might be willing to cooperate. They don't share too many religious practices - Enech is an Irish-only concept, while the Gauls' focus on sacrifice is generally eschewed by the Tuatha - so I doubt they consider themselves bosom buddies, but as occasional allies or long-lost cousins, I don't think they'd have much difficulty with one another. Few of the Nemetondevos ever made it to being worshiped on the islands, so only Cernunnos (an older god whose worship probably resembled Crom Cruach's more than the other Tuatha's) and Gobnhios are probably all that well-versed in Irish custom.

Incidentally, Gobnhios is a great place to explore the relationship between the two pantheons; he's a member of the Tuatha de Danann as Goibniu, the smith-god who forged their weapons against the Fomorians, maintains the cauldron of eternal life and killed Brigid's son Ruadan when he caught him attempting to spy and steal for the Fomorian side. He's actually far less known in Gaul, which might lead enterprising Storytellers and players to wonder what he's doing on the continent instead of at home in Ireland; perhaps he defected to escape Brigid and her overpowering grief (and possible vengeance), or for some other, more secret reason.

There's always going to be some overlap between nearby cultures; Babylonian gods were worshiped in Canaan, Canaanite gods were worshiped in Egypt, Egyptian gods were worshiped in southern Greece, and so on and so forth. We tend to view it less as a combination of those pantheons, who are usually very distinctly different in character, and more as a testament to the power of Legend and the tendency of humanity to always provide worship to deities who impress them (after all, you can't expect Joe Schominus of Gaul to know whether or not a god is a visitor or someone upon whose favor his livelihood depends).