Prefixes are word parts added to the beginning of a word to change its meaning. "Ir" is a negative prefix, meaning "not" or "the opposite of".
For example:
* Irregular - not regular
* Irrelevant - not relevant
* Irresponsible - not responsible
Therefore, the part of speech of a word containing "ir" depends on the word itself. For example:
* Irregular is an adjective.
* Irresponsible is an adjective.
* Irrelevant is an adjective.
* Irrigate is a verb.