R1b folks haven’t been IE horse riders at the first place they were seafarers and belonged to Cardium Pottery or Cardial Ware culture. They migrated from Levant (that’s why R1b Basques have no Caucasian component) by water route along seashore and made first European settlement in present day Albania (see map below).
Albania is the palace where the first European clades below R1b-L23 have appeared. It’s pretty much obvious and can be seen on the map for R1b-L151 and R1b-L51 distribution.
Moving along seashores R1b folks soon reached South France then Portugal and Britannia. From history we know them as Ibero-Ligurians. Spread and distribution of U152 clades mirror expansion of Ligures in inland France and Iberian peninsular.
Later they were captured and assimilated be IE folks.