"You are lucky."
Translation:Tá an t-ádh ort.
Tá tú ádhúil is essentially béarlachas - a literal translation of an English idiom.
ádhiúil is used as an attributive adjective (Seo an buatheoir ádhúil! - "here's the lucky winner!") but where English uses it as a predicative adjective, Irish prefers the ádh ar or ádh le construction.
It's an example where grammatical correctness is not sufficient.