irradiation has 11 distinct synonyms spread across 2 of its 6 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 condition of being exposed to radiation
a column of light (as from a beacon)
(physiology) the spread of sensory neural impulses in the cortex
the apparent enlargement of a bright object when viewed against a dark background
(Pavolvian conditioning) the elicitation of a conditioned response by stimulation similar but not identical to the original stimulus
(medicine) the treatment of disease (especially cancer) by exposure to a radioactive substance
Synonyms, antonyms and the broader and narrower relations for irradiation come from Princeton University’s WordNet. Full sources and licences.