I had posted a thread about this very issue recently. I have an Irish surname and I1-M253 with 'Ultra Norse' haplotype L813+, and matches in all the areas shown on the map you posted, but also further north in Larne and further west in Sligo. I had also been pressing the idea for a while that possibly my I1 was pre-ice bridge retreat or some other form of it but have been finally convinced that is highly unlikely. Not a problem, it's all fascinating. It does however make it at least slightly more difficult in finding matches who may also be relatives. I had joined an Irish family DNA project based in Ireland but virtually every one of the people who belong to it are R1B-M222 including those with my exact surname, and I had fully expected I would be as well. When I finally received my results I was pretty surprised. For one, not knowing a thing about all the other Haplogroups, I had never even heard of "I1". Secondly, most of my matches especially those I carry all the way through from 12-67 are located in Norway, southern Sweden and western Finland. Though I also have some in the aofrementioned map locations in Ireland. Some in Shetland, Isle of Man, Hebrides, Caithness, all down the eastern coast of England from far norther down to end of island. Also in Brittany France. Oddly enough considering the likely Viking source of my I1, I have as yet none in Orkney.