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"
"gracious, dear one"
"keen ruler, bold king"
"my God is Yahweh"
"appointed by God"
"bright, brilliant"
"brother (informal)"
"marshland meadow"
"defender of men"
"fit, shapely, graceful"
"modern elaboration of Darren"
"my covering is God; God protects"
"cross; the Holy Cross"
"bearer of Christ"
"from the city of Troy"
"of royal status"
"hunter"
"strong as a hound"
"slender youth, young warrior"
"one who tends a gate or hatchway"
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.