(1) Most of the information on the gods of Ireland and Wales is from various myths, so details can vary depending on the source; particularly in the case of Wales, mythological characters are not always identified as deities, possibly due to the stories having been written down in the post-Christian era. I am taking the broad view here and using the words “goddess” and “god” to refer to these supernatural entities.
(2) Most of the information on the Celtic gods of Britain and Gaul is archaeological or historical, a circumstance that provides its own challenges including the tendency of the Roman writers to equate all gods with their own. The Celtic gods did not comprise a pantheon in the sense that the Greek or Roman gods did–most of these deities were associated with specific tribes and/or regions, and would not have been worshipped together traditionally unless they both came from the same region, and possibly not then.
(3) On the European continent, the Gaulish and Germanic tribes can sometimes be difficult to differentiate–there was a certain amount of contact, and in some cases it is uncertain whether a deity was honored in Gaul or Germania.