ultramontane has 2 distinct synonyms spread across 2 of its 4 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.
on or relating to or characteristic of the region or peoples beyond the Alps from Italy (or north of the Alps)
“ancient transalpine Gaul was an area northwest of the Alps and included modern France and Belgium”
on the Italian or Roman side of the Alps
“ancient cisalpine Gaul included an area south and east of the Alps”
a Roman Catholic who advocates ultramontanism (supreme papal authority in matters of faith and discipline)
1 of the 4 recorded senses carries no synonym, antonym or related term, so it is not shown above. Every sense, with its definition and examples, is on the full entry for ultramontane.
Synonyms, antonyms and the broader and narrower relations for ultramontane come from Princeton University’s WordNet. Full sources and licences.