Celtic Heathenry, like Teutonic Heathenry is a Blanket Term.
Teutonic Heathenry refers to all European Forms of Heathenry.
At one point in time, there would have been a clear difference. Now, there are so few of us that we have to stand together. We start with the common ground that we are Heathens.
Some of us are going to be more Irish. Some of us are going to be more Scottish. Some of us are going to be more British. Some of us are going to be more Welsh. Some of us are going to be more Gaulish. Some are going to be more Saxon. Saxony Heathens are the line between Nordic or Germanic and the Celts.
Yes the Saxony are not quite Celtic, but they are not Quite Germanic Either. They are the Bridge.
There are subforms beyond this. These are not directly historic, but modern terms with Historical Value.
Scota - Form of Caledonia (Scottish) Druidry. Scota was the name of 2 Egyptian Princesses who settled in Scotland. This is part of why Scotland has that name. For Modern Purposes, a Scota Druid is someone who has gone through Druid Training that pays direct homage to the Celtic and Egyptian Gods.
Roman - During the Roman Occupation of the British Isles, many people were forced to worship the Roman Gods and the Celtic Gods. Sometimes Roman names were forced on Celtic Gods.
Northman Celt - Vikings settled, raided, traded, and bred with the Celtic Peoples. This creates new Stories, Lore and Law. Example Andarta was the Daughter of Nann and the Northman's God of Light. Baldr Fairhair or Lighthair was the Husband of Nann.
You bring us Historical Evidence of Crossover, we will look at it.