If the radical changes in the pronounciation it is without any doubt an irregular verb. But only in this case, if there is a change in the pronunciation of the radical, we can really speak of an irregular verb. In total there are something like 60 irregularities but there are less than 60 patterns of irregular verbs because a lot of irregular verbs are a mixture of two irregularities. Actually there are only something like 30 irregularities, for instance the diphtongization of the e (sedere => io siedo) or o becomes e (dovere => io devo). So the problem is not as big as it may appear at first glance. If you find on the internet statements which affirms that learning the irregular verbs is the most difficult aspect of learning a language you have, let' s say, not a really balanced description in front of you nose.