Michael O'Leary being incredibly rude is a key part of Ryanair's branding. If you're paying £19 for a two hour flight, you'll always have the sneaking suspicion that the airline is skimping on maintenance. O'Leary makes it absolutely clear that they save money by treating their passengers like cattle. He carefully cultivates the image of a man who utterly despises his own customers, but is far too miserly to allow an…
Which means that (i) they'll lookup the price and be upsold a package on Ryanair.com rather than finding the price and a different hotel deal on some other price comparison site and (ii) they'll still buy tickets from Ryanair.com at non-promotional rates when they're actually not the cheapest airline offering a viable route and time. And of course (iii) he can fill his flights without sharing revenues with online travel agents and other affiliates, though other low cost airlines without the reputation for taking things to extreme lengths manage that.
Although part of it's probably just his style. Not sure there was any strategy to his comments that "I don't care if no-one likes me. I'm not a cloud bunny or an aerosexual. I don't like aeroplanes. I never wanted to be a pilot like those other platoons of goons who populate the airline industry..."