This doesn't prove anything beyond the fact that both are acceptable variations.An Gobaire wrote:Deagh rannsachadh.
Leis gach 22 Le gach 3
In fact, you could argue that it supports "le gach" in a roundabout way -- we know that before the internet and text messaging, written language tended to be more conservative than spoken language. Even today, habitual forms creep in and are derided as "colloquialisms" and "errors" by editors and teachers. These aren't really errors at all -- they're slips of attention that allow the natural form to slip past the internal "censors" that insist on an unnatural form as "correct". Applying to historical linguistics, we can assume that the neologous form was fairly common if it managed to slip into an old text alongside the conservative, so-called "proper" form. It doesn't tell us definitively which was the most common form, but it proves it was a well-established form at the time.