Quotes from Joan P. Alcock

This wool may have come from the feral variety of sheep resembling the modern Soay sheep found on St Kilda, which has a brown fleece, but later selective breeding led to an improvement in quality and to whiter fleeces.

The nearest equivalents to Celtic sheep are Welsh Black Mountain and Soay sheep.

John Wild, however, suggested that a Roman breeding programme produced a sheep more akin to the Orkney sheep of South Ronaldsay that produced a thicker fleece,