It could be attempted in the 737, however, quite complicated in terms of set up and also quite attention demanding. The Ryanair SOP's certainly don't cover this kind of non-normal activity. That is why I don't believe it ever happened in the first place.
Possibly the crew had to taxy to a remote area for a crossbleed start. In which case a single engine taxy may have occured. Knowing the ramp at Shannon well however, I doubt this theory.
Every airline has a few plonkers, but I don't think an FO or Capt in Ryanair would do something like this purely to save time.