It will depend on your airline's policy. In my own case (Loganair) we are not allowed to fly for any other operator without the written permission of the boss etc. As we don't fly anywhere near 900 hours per year there is not a problem getting that permission to do a bit of instructing however I can almost guarantee that Ryanair will want 899 hours 60 minutes out of you so no chance. You can fly non-commercially of course but that won't help you.