Yield is nothing to do with it.
RYR make their profits through playing the aviation industry, not selling airline seats. Even if you sell all 189 seats on a 737-800 on a trip to Rome for £15, you make precisely £0.00 (even in euros!) once you take out staff costs, fuel costs, mortgage or leasing costs, airways charges and landing fees, handling fees, maintenance, depreciation etc.
However, you buy and lease 50 737-800s after flogging three to five years maximum use out of them and you make megabucks. Also, if you charge everybody for food, water, wheelchairs, excess baggage, inflight sales, advertising, car hire, hotels, buses and everything else they might need on one of your flights you also make £££'s.
That's Ryanair's secret. They cut cost base to the bone, then maximise revenue by anything but seat sales. They ain't never going to be BA, but they sure will prolong the myth of low-cost air travel for a few years to come, until MOL can buy Ireland and run it as his private fiefdom.
(sorry just realised he already does!!)