Hi griffks and welcome to the forum.
I'm pretty certain that they wouldn't have had a frame numbering sequence - because so many different makers built for Rory O'Brien.
If you have a Rory O'Brien frame, I'd love to see a pic of it. My Rory O'Brien frame had a grease nipple on the crank which to my knowledge is rare on bikes after the early 60s.
One way of finding an approximate date is to look at the style of the frame; at one stage there were four frames available; "Cadet" ,"DB Special " (Dave Bedwell), "Contessa" and "Championship". (Mine was a Championship frame).
I did have a copy of the Rory O'Brien 'catalogue' listing these models, but sadly, I passed it on with the bike.
IF you have an O'Brien frame and IF you want it dating, pm me and I'll give you the email address of someone who may be able to help you. He provided me with my copy of the O'Brien catalogue - which looks very crudely produced by the standards of today.
Best of luck with finding out more.