irritation has 12 distinct synonyms spread across 6 of its 7 senses. Each sense below keeps its own list, because a synonym belongs to a meaning and not to a word: swapping one sense's substitute into another sense's sentence is how a thesaurus makes writing worse. WordNet records no antonym for it — antonymy is a relation between word forms rather than concepts, so it is deliberately sparse.
Counts are distinct word forms across every sense. A synonym that appears under three senses is counted once here and shown three times below, under each sense it belongs to.
the psychological state of being irritated or annoyed
(pathology) abnormal sensitivity to stimulation
“any food produced irritation of the stomach”
the neural or electrical arousal of an organ or muscle or gland
an uncomfortable feeling of mental painfulness or distress
unfriendly behavior that causes anger or resentment
the act of troubling or annoying someone
Synonyms, antonyms and the broader and narrower relations for irritation come from Princeton University’s WordNet. Full sources and licences.