I know a few for Welsh:
AURELIUS - ? There is the Modern Welsh euryll
"golden"
AMBROSIUS - Old Welsh Embreis (Modern
Emrys)
CONSTANTINUS - Old Welsh Custenhin
CAESAR - Modern Welsh Cesar
VALERIUS - should be Modern Welsh *Gwalyr or
*Gwelyr
VALERIANUS - ? Perhaps *Gwalyrion /
*Gwelyrion
CLAUDIUS - ? We should expect Old Welsh *Cloud
(Modern Welsh *Cleudd)
JULIUS - ? should be Modern Welsh *Iul (just as
Latin iudex gave Welsh iudd)
JULIA - ? should be Modern Welsh *Iul
MARCIUS - ? Perhaps *Meirch. There is the closely
related Latin Marcus, which gave Old Welsh March (which was confused with the
native Welsh march "horse"), as well as Marcianus, which gave Welsh
Meirchion.
CAIUS = Welsh Cei (Sir Kay)?
- Possibly (We should expect *Cai), but there might be a native Celtic
derivation as well (perhaps Brythonic *cagio- "encloser" or a loan from
Irish).
-C. Gwinn