They were proto-Celts, and in small quantities. The major celtic incursions came after 500BC and were major invasions, not just little trickles. The following sort of basic celtic history is available everywhere on the net:
"The agricultural, bronze-using Urnfields, who can be thought of as proto-Celts, existed from roughly 1300-700 B.C.E.. They were succeeded by the Hallstatt culture around 700-600 B.C.E., which is distinguished by the use of iron and a change in burial practices. Around 500 B.C.E., the La Tene culture appeared, signifying a shift of power within the loci of Celtic groups, as well as important contacts of trade and artistic production. At this point there are obvious influences of the Mediterranean world in Celtic art and metalwork; presumably, Mediterranean ideas and mythologies also found their way westward."
A language can change in two generations. Who knows where it came from. I don't know where and don't see the importance. The experience in America proves that language is a poor indicator of genetics.
When and where did the language change?
Dunno. Don't care.
Languages of the Celtic family used to be spread all across Europe, especially before the Romans and later the Germans restricted them to mostly the British Isles. There were Celtic Gauls in North Italy in the 300s BC, for instance. Across the Alps in France. There are differences in language across that sweep but they aren't as big as the differences between, say, Welsh Gaelic and Anglo-Saxon. It doesn't look like much change happened across that part of the journey.
Then they learned them from them, I guess.
But in fact, there were identifiably Celtic languages, all highly related to each other, in Eastern Europe and Asia Minor in historical times. So it doesn't look as if the change from Hebrew/Semitic happened on that part of "the journey" either. So when did the change happen and why didn't it keep happening thereafter?
I have no idea. And I have no idea why the English language has changed so much in the last 500 years It happens.
You're not telling me a coherent story here.
You're point is meaningless. Language is a poor indicator of genetics as the experience in America proves.