It seems to me that Irish treats the definite article more similarly to Romance languages than English. In both, one studies the maths and the history, and one travels to the Italy and to the Greece. This translation sounds more right to me than including the in the English.
Your previous complaint was about "There is a good explanation in the new book", which DOES have "a" in it.
Irish doesn't use indefinite articles. English does use indefinite articles, and in most cases, those indefinite articles are not optional in English, even though they don't exist in Irish.