immaterial has 6 distinct synonyms spread across 4 of its 5 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 material.
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.
of no importance or relevance especially to a law case
“an objection that is immaterial after the fact”
not pertinent to the matter under consideration
“an issue extraneous to the debate”
(often followed by `to') lacking importance; not mattering one way or the other
“whether you choose to do it or not is a matter that is quite immaterial (or indifferent)”
Synonyms, antonyms and the broader and narrower relations for immaterial come from Princeton University’s WordNet. Full sources and licences.