It may well have been 1659 or 1660 (Volvo B10M/ Plaxton Excalibur) working Eurolines work (but I think these are in White with Eurolines logos) or one of the other buses based in Stranrar.
It is also possible that it was on Tour work as some (at Least!) of the depots use the Mark IV Goldliners for tour work. Derry use 1656/7 for only tour work. An interesting fact is that 1657 is the only one of these two buses to have seen normal service work, as it was used on the 273 to Dublin, in which, on this journey it sustained a damaged wing mirror on its first, last and only service run.