Re-reading your initial post... the Boreray wether is presumeably horned, albeit smaller than an entire's headgear would be, and the "mule tup" is presumeably polled. So if the wether was protective of the ewes, it could have made it difficult for the tup to mate, yes. But surely you would have observed fighting?
(I put "mule tup" in quotes, because "mule tup" isn't an animal that would normally be used for breeding. Tups would normally be pure bred, or specific crosses of similar types. The outcomes would very variable using a mule as a tup.)