An interesting observation in this study is that within the “northern” European population group, individuals of Irish descent showed substantial differences in substructure compared to participants of Scandinavian, Central, and Eastern Europe descent. It also appears that United Kingdom individuals were intermediate between the other non-Irish groups and those of Irish descent further supporting an east/west gradient (Table 2). However, the later observation is based on small numbers of individuals (six 4GP United Kingdom individuals). It is unclear whether these relationships may reflect remnants of early populations including differences in Mesolithic or Neolithic contributions to the Irish population 5,000–6,000 years ago [28], or later Celtic contributions. An extensive Neolithic contribution from the Iberian peninsular is consistent with Irish archeological information but it is unknown whether this population group survived [28,29]. As discussed above, it is difficult to determine the relationship between certain population groups and the suggestion of a cline extending from the Spanish to Irish population is tenuous based on the current data. However, we note that there is modest support for such a cline in both PC2 and PC3 (Figures 2C and 6)
