He died from cancer should also be correct
"He died of cancer" is correct English, "He died of a cancer" is weird.
It sounds alright to me, in some cases (such as not wanting to specify the type of cancer).
Sure, but I don't think "he died of cancer" should be marked wrong. How could you even distinguish the two in Irish? There's no "a".
Oh, I agree the former shouldn't be marked wrong. It's definitely the most common in my dialect.
It’s accepted as correct as of 2015-11-07.
They could only be distinguished in Irish by context.
Is there a difference between saying >den ailse< (as in a previously-presented example) and >d'ailse< (as here)?
Yes. "den" is a contraction of "de an" so in this case, there is a definite article (the). In context, I think that you might have identified what kind of cancer the person had already and now you are adding that they died of the cancer.