An interesting bit of research released this last week shows that almost half ot eh Ashkenazi jewish population is descended from four mothers who most likely lived around 1000 years ago. Science daily also reports:
Non-Ashkenazi Jews also carry low frequencies of these distinct mtDNA types, providing evidence of shared maternal ancestry of Ashkenazi and non-Ashkenazi Jews.
You might have also seen this report on a similar sounding bit of research, evincing that some 20% of northern Irishmen carry a genetic marker indicating a shared many-greats-grandfather dating back to the 5th century?