I don't have a definite answer, nor indeed the experience of ttc0169, but, my fondest memories of oil traffic is the A class locos hauling the oil tank wagons into Ballinacourty in the late 70's, early 80's. From memory, there was no barrier wagon on these trains, but, it is 30 years ago now, and I may be wrong.
Also, on the back cover of the IRRS journal, no.112 from June 1990, there is a photo of a A class and oil train passing the Curragh in Jan.1979. There is no barrier wagon on the train, but, the info about the photo states that the train is empty. However, I'd imagine the barrier wagon would still be present, if it was in use......