Here's why it's considered irregular:
* Regular verbs form their past tense and past participle by adding "-ed" (e.g., walk, walked, walked).
* Irregular verbs have unique forms that don't follow this pattern. "Write" doesn't follow the "-ed" rule; its past tense is "wrote" and its past participle is "written".
So, while "written" itself isn't a verb, it is a form of an irregular verb.