Hilarity rhymes with:
celerity, clarity, irregularity, polarity, popularity, regularity, similarity, celerity, clarity, dexterity, disparity, familiarity, geraghty, irregularity, parity, peculiarity, polarity, popularity, posterity, prosperity, rarity, regularity, similarity, sincerity, solidarity, temerity, unfamiliarity, verity
Hilarity sounds like:
hallward, hellard, hillard, hilliard, hillyard, hilyard, hollered, holroyd, holwerda