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"
"who is like God?"
"friend, companion"
"gray, silver-haired"
"from Luca, of the light"
"continuous improvement"
"descendant of the gentle one"
"golden chamber, bay window"
"ruler of the world"
"one who wrangles livestock; a cowboy"
"son of Morris/Maurice"
"one who follows at the heel"
"forest, wood, timber"
"may God protect"
"God is the Lord"
"free man"
"from the long shore"
"to shine, emit light"
"hill, eminence (via Brian)"
"son of Hendrik"
"son of Jacob"
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.