There is no reason why he couldn't dip into his reserves, but it's highly unlikely he will when he feels he doesn't have to and he hasn't had to elsewhere. MO'L's mantra is "if someone else can pay why should I".
I am sure that Derry City Council and the UK and Irish governments are concerned that Ryanair could still decide to pull the plug even if they invest in the airport, however, they will be looking at any additional services or third party aircraft maintenance work which may become possible through extending the runway. It will be interesting to see how this plays out.