For fun, here is a brief summary of the consensus that Kardu, haithabu, and I seem to be coming to about the history of I2c (obviously not all of this is certain and this is subject to revision):
The History of I2c
I2c is a subclade of Haplogroup I, the only surviving Y-DNA haplogroup that can be said with high confidence to have been in Europe since the Paleolithic. By around the beginning of the Neolithic in Europe, I2c had bottlenecked into three major branches, all of which did not expand until significantly later, remaining minority clades in populations near the Rhine.
The first to expand was the "C" branch. Although perhaps the least common nowadays, it diversified within Western Europe nearly 4000 years ago, probably around the Middle Rhine, from which it became a minority clade in some tribes, mostly Celtic and maybe some Germanic. It is still a trace haplogroup everywhere it is found.
The "A" branch waited another 1000 years or so, but followed a similar pattern to "C." Having a strong affinity to I2a2b, it was apparently a tiny trace minority clade within Urnfield Culture before having some minor success in expanding with the Iron Age Celts. Minor expansions on the Brythonic area of Scotland and the Emmental in Switzerland made it perhaps surpass "C" in numbers.
Both "A" and "C" are dwarfed in numbers by "B." Although it expanded late, around the same time as "A," and shares a Western center of diversity, probably not too far from the Rhine, most of the expansion of "B" happened in Eastern Europe and Asia. Some minor gene flow had some "B" carriers end up in Southeastern Europe as a small minority, where some stayed and others, as part of the Phrygian/Mushki migrations, ended up in Anatolia. Those who remained in Southeastern Europe remained a small minority, and maybe even bottlenecked further, before finding some minor success within the Venetian Republic, especially in Crete, and also later with a Jewish family who expanded significantly Northward. Those who moved onto Anatolia became even luckier, as "B" found itself into the local nobility, expanding accordingly and becoming perhaps the most common I subclade in Asia.
