Hmm now that’s actually a hard one since most stuff doesn’t deal with any of that much to my dismay. Well I can’t think of any new movies or shows, only things like Starz’ Camelot, Tristan and Isolde 2006, the Book of Kells is animated but celtic and cute and has great animation, the 1998 Merlin miniseries, there is a movie called Druids from 2001, Boudicca from 2003, oh and Centurion which is more recent and about roman soldiers who get stuck in pagan Scotland, the eagle 2011,. Lost girl includes a lot of mythology some that is celtic and pagan but that’s not the main focus of it. There is a more extensive list on imdb.
Books probably more likely:
Daughter of Ireland by Juliene Osborne McKnight - also has Song of Ireland and Bright Sword of Ireland
Any of the Sevenwaters Series books or Heart’s blood by Juliet Marillier
The twilight of Avalon series
Daughter of Lir by Diana Norman
Raven flight, swan maiden, and the white mare trilogy by Jules Watson
The princes of ireland
Marion Zimmer Bradley has pagany stuff set in Britain, like the forest house, ancestors of Avalon, etc and mists of Avalon of course
Peter treymayne writes a murder mystery series set in 7th c Ireland
Morgan Llywelyn has some books such as Druids and Lion of Ireland
Confessions of a Pagan nun by Kate Horsley
The circle and the cross series which I really want but haven’t been able to get.
And Arthurian stuff I would say the series by Nancy Mckenzie, Helen hollick, and the Mary Stewart trilogy. I have many more Arthurian ones too if I looked at my kobo. Also check goodreads list of celtic fiction they have a lot up there.
I hope you can find something in there that helps! And thanks for asking me :D