How this happened needs to be put in some context, for the benefit of younger readers.

From the time that Atlanteans first appeared in Dublin in 1966, CIE were naturally keen for buses to be OPO - after all, in UK that was normal, and buses were designed specifically to allow them operate as OPO. Single deckers were already OPO on most routes, following a series of industrial disputes (both Dublin and Provincial) in the mid-60s.

Eventually, an agreement in 1986 paved the way for OPO double deckers in Dublin. But the catch was that every route had to be either OPO or TPO (two person operated), and conversion could only happen if there was 100% agreement among the marked-in drivers and conductors. They could either go OPO, or move to other routes. Pressure from their colleagues (after all, those going OPO would be paid a lot more!) persuaded most to convert or move, and by late 1988 CIE/Dublin Bus had achieved their initial target of conversion of 75% of routes.

Next step was to tackle the remaining 25%, and as there was now only one TPO route left in many garages, and as it was going to be difficult to get 100% agreement, a proposal came to have mixed OPO/TPO on the remaining routes. This was agreed, with guarantees for remaining TPO drivers and conductors, and all routes went mixed OPO/TPO in summer 1990. These routes were 3, 11, 51, 67, 42b, 20b, 41, 33, 84, 22, 15. All these routes got new timetables in July 1990, except the 33, which was delayed to September 1990 due to a dispute over Sunday rosters.

In the meantime, while Dublin Bus was trying to achieve this agreement, the number of conductors was declining. The 51 was not the only route to suffer. I distinctly remember the late Eamon McArthur telling me of nearly half the 42bs being off the road due no conductors. It was not a a happy time if you happened to live on one of those routes. Of course, there was no information to passengers, you could not admit in advance that some buses may not operate!

The mixed service time was very confusing for passengers. Some buses operated with "conductor on duty" stickers. Also around 1987/88, before the 16 was converted, nearly all the routes through Drumcondra were still TPO - 3, 11, 16, 41 and 33. But there was a small exception - the 33b that ran twice a day into the city. This caused great confusion as passengers did not expect a bus on this corridor to be OPO.