Irregular verbs do not follow a regular pattern of conjugation.
How many irregular verbs do we have?
About 4000 verbs belong to the first conjugation and only four of them are irregular: andare, dare, fare, stare.
About 500 verbs belong to the second conjugation. Here, we have about 25 irregular verbs in various tenses. The most important are: essere, avere, dovere, potere, volere, sapere, piacere, vedere.
About half of the second conjugation verbs are irregular in the past participle. So, you need to learn them if you want to use compound tenses!
Many verbs are also irregular in the simple past, which however is rarely used in spoken language.
A few verbs belonging to the third conjugation are irregular. The most important are: dire, uscire, venire.