There are a couple of new books out that are attempting to trace the histories of the peoples of the British Isles using DNA.

May I suggest you find a copy of;

Saxons, Vikings and Celts
The genetic roots of Britain and Ireland
by Bryan Sykes
Paperback version published by Bantam Press 2006
ISBN 978-0393062687

or

The Tribes of Britain
Who are we and where did we come from
by David Miles
Published by Phoenix Press 2007
ISBN 978-0753817995

I have just finished the first of these and there are some surprises. Bryan Sykes is one of the most respected DNA researchers today. After analyzing his data he makes the postulation that it seems the current interest in Celtic culture is due to WWII. Prior to that people of the isles considered themselves more aligned with the peoples of Germany.
His research can find no "Lost Tribes", Celts as a race, or peoples other than the Picts who may have inhabited the Isles prior to the arrival of the H haploid group arriving via the Atlantic seaboard route from Spain.
As more mDNA and Y chromosome research is done I think we may have to re-think all of our roots.

For more information may I suggest you check out "The Genographic Project" sponsored by The National Geographic Society and IBM. Bobbie and I got each other DNA sampling kilts for Christmas and her family line is already plotted. Mine should be finished in a week or two.