We. Told. You. So.
First a word on the Fabric, this is the St Ninian’s Tartan presented to the Holy Father on his visit to the United Kingdom; it is thus the Papal Tartan, and as such to be worn by the Sovereign Pontiff or a member of his staff. This is as much as we know; the rest of this post is pure speculation.
So here we find ourselves sat before a hat that golfing priests might well fight over. Our best guess is that it might be worn by a priest of the Papal Household on a trip to St Andrews should he find himself fancying a cheeky nine holes. Should such a moment of excitement fall on a Spanish feast day, naturally we have that covered also.