Technically, the Hammer (Mjolnir) is Thor’s symbol, but the World Tree / Tree of Life is a big shared symbol. That would work well anywhere.
For the individual shrines, you may want to work out which deities (if any) you want to specifically honor, then include their symbols. I can think of some general ones off the top of my head though.
- World Tree
- Bifrost (Rainbow Bridge)
- The World Serpent, Jormungandr
- Ravens (Odin’s, yes, but also messengers)
- Futhark runes
- Well (Ymir’s Well, the font of knowledge)
- Thor’s Hammer (Mjolnir is a bit deity-specific, but very recognizable, and Thor is the protector of humanity, so it’s a good thing to have around)
- Tree of Life
- Cauldron (figures in many stories)
- Salmon (fish of wisdom)
- Shamrock (three- or four-leafed clover)
Also, Norse and Celtic knotwork is very similar in design, so you could probably find some neat pieces with the right aesthetic for a joint shrine or for separate ones. Plus, some of those listed elements are similar enough that they can pull double symbolic duty if need be (i.e. a bowl for the Cauldron and the Well).
Celts and Heathens, feel free to chime in.
There is no mention of a “world tree” or “tree of life” at all in Irish or other Celtic cultures. There might have been tree shrines and or temples. The Irish honored the three realms of Land, Sea, and Sky. Which would be good imagery to work with. Knotwork is only Norse and Germanic, the Irish did not use it until contact with Anglo-Saxons, and Norse peoples. And most of that art is from post Christianization of Ireland. The pagan Irish used La Téne art work like their Celtic cousins.