Many believe this group crossed over from Africa and arrived in the British Isles from Iberia (Spain) rather than the steppes.

dna.ancestry.com further noted:

"R1b3 has its origins about 11,800 years ago. Within the British Isles, a genetic pattern called Atlantic Modal Hallotype (AMH) features greatly among the Irish and Welsh. Some researchers consider this haplotype to be representative of the early Celtic migrations. Many believe this group crossed over from Africa and arrived in the British Isles from Iberia (Spain) rather than the steppes.

http://users.reachone.com/jfsmith/Origin.html