Brennen sits next to 4 phonetic neighbors — Brendan, Brennan, and Brandon are the closest matches by sound. Most share Brennen's Irish-rooted cluster, so the rhythm and vowel shape carry across the list.
You searched for names like
"prince, king"
"strong ruler"
"hardworking; powerful"
"of royal status"
"fighter, battle"
"modern American creation, gentle (from Kevin root)"
"free man"
"lion man"
"grey home, gravelly homestead"
"sky blue, cerulean"
"God has been gracious"
"thin, lean"
"son of Robin, son of Robert"
"like the sun"
"deep blood-red color"
"beautiful, good, noble"
"he will enlighten"
"God has given"
"golden chamber, bay window"
"ancient, enduring, long-standing"
"brother (informal)"
Similar sound: These names share phonetic patterns with Brennen — similar vowel sounds, consonant clusters, or rhythm.
Shared roots: Several names on this list have Welsh/Celtic origins, giving them the same cultural heritage as Brennen.
Rhythm: Brennen is a 2-syllable name. Names with the same syllable count tend to feel interchangeable when you're narrowing your list.