"cúthalach" = shy, not cúthail
cúthail works as well. FGB shows duine cúthail as 'a shy person'
The FGB shows cúthal(ach) and cúthalta as variants of cúthail. Dinneen also listed cúthail rather than the other variants.
There's another word for shy isn't there?
According to foclóir.ie, there's faiteach and cotúil.