"Good morning Iain, it is a good morning."
Translation:Madainn mhath Iain, tha deagh mhadainn ann.
It could (’s e deagh mhadainn a th’ ann would be more literal it is a good morning, and also OK).
Gaelic tha deagh mhadainn ann does not literally mean it is a good morning but rather something like there is a good morning there, in general existence.
Grammatically the sentences are indeed different: in Gaelic the subject is deagh mhadainn a good morning and the predicate is a dummy ann there, in general existence; while in the English sentence the subject is a dummy it meaning general condition around and the predicate is a good morning. But the meaning they convey: equating the general current condition to a good morning, and their usage: describe the condition around when greeting somebody, are the same.
That’s just how you say something like it is a good morning! in Gaelic. Similarly it’s a fine day would typically be tha latha brèagha ann, etc. But latha brèagha a th’ ann (or longer ’s e latha brèagha a th’ ann) is also possible.
See also a similar discussion under "Good morning Calum, it is a good morning".