shamrock has 9 distinct synonyms spread across 3 of its 3 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.
creeping European clover having white to pink flowers and bright green leaves; naturalized in United States; widely grown for forage
Eurasian plant with heart-shaped trifoliate leaves and white purple-veined flowers
clover native to Ireland with yellowish flowers; often considered the true or original shamrock
Synonyms, antonyms and the broader and narrower relations for shamrock come from Princeton University’s WordNet. Full sources and licences.