Una sits next to 4 phonetic neighbors — Luna, Nona, and Ana are the closest matches by sound. Most share Una's Irish-rooted cluster, so the rhythm and vowel shape carry across the list.
You searched for names like
"one, unity"
"high hill"
"lass, maiden"
"song thrush (bird)"
"gift of God's grace"
"from the Welsh elements dyfn (deep) and related roots"
"dweller near the oak trees"
"diminutive of Mary"
"headland, promontory, cape"
"dove"
"God is gracious"
"lord, master (feminine)"
"envious (alternative etymology)"
"slender, beautiful"
"brightness, beauty"
"dedicated to Jupiter"
"daughter of the fire-born"
"waterfall, stream"
"flax clearing"
"olive tree"
Similar sound: These names share phonetic patterns with Una — similar vowel sounds, consonant clusters, or rhythm.
Shared roots: Several names on this list have Latin origins, giving them the same cultural heritage as Una.
Rhythm: Una is a 2-syllable name. Names with the same syllable count tend to feel interchangeable when you're narrowing your list.