I am not convinced Germanics and Celts are distinct genetic groups. These may have been random arbitrary classifications made by Romans. Sort of like how Americans call everyone in the Middle East "arab" even though their are many ethnic groups there.
Also the idea that "Celtic" people in The British Isles would be the same as "Celtic" people in France doesn't make sense. Would you say that English speaking Americans are the same genetically as English speaking British people? Then why would Celtic speaking Britons have genetic sameness with Celtic speaking Gauls(pre-Roman & pre-Frank France)?
And were the Gauls even majority Celtic? From what I understand there were a variety of tribes in there. But I have not even analyzed the languages, are Celtic and Germanic languages even very distinct? If they are distinct language groups, then the only thing we can say are Celtic and Germanic would be languages. That would not be evidence of Celtic and Germanic genetics.
And if you look at Ireland they had Norse migration during the Viking era. But what makes you think they would have only had Norse migration then? Ireland could have been experiencing migrations of Norse and others for thousands of years prior to the Viking Age. Europeans have been in Europe and moving around well before recorded history. That would mean you would have all sorts of people moving between tribes and tribes moving around to different territories.
The idea of distinct Germanic and Celtic genetic groups doesn't make sense. As for linguistics I am not sure about that. You can have certain genetic clusters within Ireland and England and France, and etc. But the idea of Celtic or Germanic DNA does not make sense to me.
There is Western European DNA groupings. But I wouldn't call those groupings Celtic or Germanic. I might define those groupings according to land, France, Ireland, Italy, England, etc.