"Tá deacracht aige leis an léacht."
Translation:He has a difficulty with the lecture.
Sometimes it is clear cut when a phrase is a direct translation, but quite often it is a difficult call to make. In the case of the latter, it is best to defer to credible sources.
In this specific case, the exact phrase has been included in the NEID and is supported there by several examples (please see the link below). Whilst I do not doubt that there are other ways to express this, I have to accept this phrase's validity, as it has been deemed acceptable by academics whose individual and collective knowledge of the language is far greater than my own.
If you feel this is unreasonable, I am sure that the NEID would welcome your contribution (please direct feedback to firstname.lastname@example.org).
"deacracht" is a noun - "a difficulty".
"deacair" is usually an adjective, but it is sometimes used as where you might normally use a noun - particularly in a copular phrase like "Is deacair teanga nua a fhoghlaim" where you might expect two nouns in Irish, even though "difficult" is an adjective in the English sentence (It is difficult to learn a new language).