Is it more natural in Irish to say "Is mór an domhan é" rather than "Tá an domhan mór"?
With a predicative adjective, bí is nearly always used rather than is. Is mór an domhan é is a different sentence that uses an emphasized attributive adjective — “It’s a big world.” (Despite the an, the noun is translated as indefinite.)
In the 1961 edition of Teach Yourself Irish, which focused on the Irish of west Munster, part (d) of Lesson VIII notes the following:
In such a sentence as “good advice is a great help”, the normal construction is permissible: is cúnamh mór comhairle mhaith ; but when there is emphasis on the predicate, the common usage is to make the adjective predicative and the noun definite, so that the logical subject appears in apposition: is mór an cúnamh comhairle mhaith. Thus the common form is: is maith an rud é “it is a good thing”, rather than rud maith is ea é. This form gives greater emphasis than (c).
Learn the following three sentences by heart:
is breá an lá é “it is a fine day”
is mór an trua é “it is a great pity”
is láidir na fir iad “they are strong men”.
It seems to me that this structure is accepted in Munster Irish.