Mirth sits next to 2 phonetic neighbors — Mirta and Berth are the closest matches by sound. Most share Mirth's English-origin cluster, so the rhythm and vowel shape carry across the list.
You searched for names like
"merriment, joyful laughter"
"joy, happiness"
"grove of poplar trees, tree-lined avenue"
"Olive tree; peace; a variant of Olivia"
"lily flower, night"
"spring-born, of the spring season"
"blooming, goddess of flowers"
"bright, shining (via Bertha root)"
"sea of bitterness"
"bold, to have courage"
"God is gracious"
"gift of God"
Similar sound: These names share phonetic patterns with Mirth — similar vowel sounds, consonant clusters, or rhythm.
Shared roots: Several names on this list have Old English origins, giving them the same cultural heritage as Mirth.
Rhythm: Mirth is a 1-syllable name. Names with the same syllable count tend to feel interchangeable when you're narrowing your list.