While the discussion was not quite as specific as some of your questions, the general topic of Celtic migration to Britain arose a few days ago, with a comment from /u/amusiclistener in response to "Did the Celtic peoples "arrive" in Britain, or did Celtic culture just spread to the bronze-age inhabitants of the island?" which discusses some of the current understanding of this subject, and includes an excellent bibliography from which Facing the Ocean would be a particular recommendation on this subject.

The Atlantic fringe of Europe has a rich and ancient archaeological history, and was a region of significant trade and development. Depending on the responses, you might consider asking some of these particular questions with a broader scope. "What did trade and settlement look like around the Atlantic in pre-history?" as an example, since it might cast a wider net across subject expertise. The earliest period of Celtic language and cultural development is not well understood, even less so in Ireland and Britain, and it is not always clear when the known archaeological record in a region can be correlated to particular populations.