"As man moved out of Africa they moved to areas, grew in size then some would break with the group and move into a new area. Each moving group would become the founders in the new area taking with them a subset of the genes from the African population. Time and distance created mutations in SNPs and STRs, called genetic drift, in the process of natural selection of the best traits meeting the relevant environmental conditions."
The Atlantic Modal Haplogroup, R1b, is the most common haplogroup in Atlantic Europe. It is further refined in the Western Atlantic Modal Haplogroup which exists in high or very high frequencies in all of Western Europe from Spain in the south to the British Isles and western Scandinavia in the north.
Initially it was thought that if you are within a genetic distance of any identified R-L21 haplogroup, you could assume that you are most likely in the haplogroup. FTDNA measures their member Close matches by counting mutations from member to member. On closer scrutiny I have found that counting mutations puts an individual in a pool that may or may not be within the same haplogroup or sub branch. FTDNA uses a statistical analysis for the probability that these are, in fact a members close matches.
Every male on earth is in a single phylogenetic tree. A person may have the same STR signature as another but be identified as belonging to another group by SNPs which occur randomly. Each male descendant will inherit these SNPs. For the South Irish, the unique SNP is CTS4466+. CTS3974 seems to occur whenever CTS4466+ occurs.