A question about the Celts

What is it like when the Celts emerged? and why is there such a huge genetic difference between the Celts in Europe?
For comparation:
Iberians (Pre-Celts) -
Attachment 115384
Celtiberians (Celts but with influences of the Iberians and maybe few from proto-basques?)
Attachment 115385
Cotini (Celts with very few influences )
Attachment 115385

Were the Celts really an ethnic-genetic group or was it just a shared culture between different peoples? and when did the Celts split between the mainlanders and the islanders? I'm quite a layman on the subject, so any help or reading indication will be welcome.:scratch:

and with a bonus question, North Spain (Galicia, Asturias and etc) and Norte Portugal (Minho, Trás-dos-Montes) really have Celtic influences (at least genetically?):)