irreverent has 4 distinct synonyms spread across 2 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. Its one recorded opposite is reverent.
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.
showing lack of due respect or veneration
“irreverent scholars mocking sacred things”
characterized by a lightly pert and exuberant quality
“a certain irreverent gaiety and ease of manner”
not revering god
Synonyms, antonyms and the broader and narrower relations for irreverent come from Princeton University’s WordNet. Full sources and licences.