Hmmm, I understand why Douglas gave that as a reason, but I personally think she's wrong. This page on Irene is quite interesting, and the opera buff who wrote treats being a contralto prima donna as being completely possible, and even picks out possible roles she could have performed. (He even gives a fascinating theory for why her opera career ended so early that further reinforces the idea that she was just not a soprano.)

Seeing as Doyle was quite up to date on music of the time, I tend to side with him rather than Douglas.