They also have no participles and no infinitive form. They are radically different, grammatically, from other English verbs. I did mention that they "inflect only for the past tense".

It's hard to call them irregular as to the third-person present singular; there is a rule there and all of them follow it. They're just different from non-modal verbs.

You could fairly call shall and will irregular in their negative forms shan't and won't.

The passive marker get is irregular in failing to be an auxiliary verb, unlike all modals, both aspect markers, and the passive marker be.