WordNet records no synonym and no antonym for irrigation. It is the only member of each of its synsets, which is common for technical, taxonomic and single-sense vocabulary — there is no other English word that means the same thing. The broader and narrower terms below are the useful relations in that case.
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.
supplying dry land with water by means of ditches etc
(medicine) cleaning a wound or body organ by flushing or washing out with water or a medicated solution
Synonyms, antonyms and the broader and narrower relations for irrigation come from Princeton University’s WordNet. Full sources and licences.