I’m not sure if this would suit at all, but I’ll supply some hypothetical Pritmitive Irish forms of these names here just to consider
Gabriel: borrowed into Latin from Greek from Hebrew. So instantly very hard to render in Irish (Semitic > Hellenic > Italic > Celtic)! Old Irish simply retains Latin form,
Gabriel. The clusted -ie- isn’t native to irish, the closest being either -ei- > -é-, or -e- > -ia-, hence the modern
Gaibrial. A hypothetical (and in no way historical) Celtic form would by
*Gabirelos >
*GABIRELAS > OIr.
*Gaibrel > Mod.
*Gaibhreal, which doesn’t have the vowel cluster at all, though.
Julia: from Latin
Iūlia, a feminine form of the gens (family) name
Iūlius. It is supposedly a contraction of the Old Latin
Iovilios “descending from Jove/Jupiter”. So lets take an archaic feminine form
Iovilia and borrow across as a feminine iā-stem, perhaps like
IULIA in a Latin-like form (attested rendition of Latin
iu- in Ogham, see
IUSTI for genitive of Latin
Iustus < PIt.
*jowestos), or more natural
IULE (see reduction of iā:
*mailyās >
MAILE- > OIr.
Máel- > Mod.Ir.
Maol-).
Kiera: the actual Ogham inscription CERAN(I) (genitive of
Cérán >
Cíarán, modern
Ciarán) exists, and the nominative form would be CERAGNAS (later CERAN(N)AS > Cérán > Cíarán > Ciarán). The name Ciara (> Kiera/Keira) is a feminine form of
Cíar (mod.
Ciar) from the adjective
ciar < OIr:
cíar <
cér < PIr:
*CERAS < PC:
*ceros < PIE:
*ḱey-ro-. I believe the addition of -a to feminize names was inherited from Latin nomenclature, but the feminine ending -ā of Celtic has the same origin and function in nouns/adjectives. So the hypothetical feminine adjective “dark” in Celtic should be
*cerā, which yeilds
*CERA in Primitive Irish (Ogham). This is the attested name of Saint Ciara (Cera) from 7th century, also.
Father: Mod.
athair < Old.
ath(a)ir < PC:
ɸatīr < PIE:
ph₂tḗr, with no attested Primitive Irish/Ogham form, but would presumably be
AT(T)IR –
Stair na Gaeilge gives
*atīr.
So recap:
Gabriel:
GABIRELAS pronounced /gav(i)relah/
Julia:
IULIA or
IULEKiera:
CERAFather:
ATIR or
ATTIR pronounced /aθi:r/
In conclusion, both Gabirel and Julia are very hard to render in Ogham, while Kiera/Father can be theorised with more certainty….
Therefore, it might be best just to transcribe Gabriel/Julia as GABRIEL and IULIA...