When I was based at HMS Dolphin the radio ops and cooks lived in a block that had been at one time stables for the RHA which were subsequently condemned for horses. They put submariners in there instead. Anyway, the approach to the mess was via ramps, for the horses to climb to their beds, which were and still are very similar to the monkey steps but a bit longer. So..... the donkey theory is pretty good, even though they were and still are the "Monkey Steps" to me.
Birkenhead........ God's own Room 101.