Here are some synonyms and antonyms for the word "irrelevant":

Synonyms:

* Unrelated

* Unimportant

* Insignificant

* Extraneous

* Beside the point

* Off-topic

* Non-essential

* Meaningless

* Frivolous

Antonyms:

* Relevant

* Important

* Significant

* Essential

* Pertinent

* Applicable

* Material

* Meaningful

* Valuable

The best synonym or antonym to use will depend on the specific context of your sentence.