If you aren't going to try, there's not much to say.
But even so, we have centuries to work with in order to get them to Europe.
Not at all! They're well established in Central and Northern Europe well in advance of the Roman expansion. The given date of 610 for an emergence in Eastern europe is probably pushing it the wrong way.
The original wave of Celtic immigrants to the British Isles are called the q-Celts and spoke Goidelic. It is not known exactly when this immigration occurred but it may be placed somtime in the window of 2000 to 1200 BC.See what I mean about this not working?From The Celts.
You've been introduced to the facts of any effort I would put out already, on this thread at least.
Your quote doesn't say where the original Celts came from. All people come from somewhere. When Saint Paul went up into Europe and built his churches, he found his people (Isrealites) already there. A thousand years before the Nothern tribes escaped, Israelites took their hats out of Egypt and didn't go home. The only other place they could have gone was Africa and Europe. That matches your time period.
What can a bunch of people do in a vast area in a milenium? A lot wouldn't you suppose?
So we have Old Celts and New Celts. Apparently the common identification as "celt" indicates the two groups were somehow related to the naked eye.
The Northern Kingdom tribes left their captivity around 610ish BC. They were just south of the Causcaus mountains. and northerly was the direction away from their oppressors. It doesn't take a rocket scientist.
But anyway, all this is a real page-turner, but the question remains, who are the descendents of the lost tribes? In order for them to be as the stars in the sky, they would have had to be a major population in any sequential period in history, including this one.
What other candidates?