Sullivan sits next to 4 phonetic neighbors — Callum, Sullivan, and Donovan are the closest matches by sound. Most share Sullivan's Irish-rooted cluster, so the rhythm and vowel shape carry across the list.
You searched for names like
"descendant of the dark one"
"brave in battle"
"fortunate, good"
"valiant, brave"
"from the valley of water"
"protector of men"
"little saint"
"deer lover, champion warrior"
"Poecc's estate; Poecc's settlement"
"man of peace"
"helper, one who helps"
"dove"
"fortified hill"
"wine stone"
"tile maker, brick layer"
"descendant of Ceallachán"
"lord, master"
"wise protector"
"red-haired, little red one"
"bold counsel"
Similar sound: These names share phonetic patterns with Sullivan — similar vowel sounds, consonant clusters, or rhythm.
Shared roots: Several names on this list have Irish Gaelic origins, giving them the same cultural heritage as Sullivan.
Rhythm: Sullivan is a 3-syllable name. Names with the same syllable count tend to feel interchangeable when you're narrowing your list.