Judging by the other examples and smrch's comment above, the space here is probably a typo. EDIT: it's not a typo, I got m'aintín and m' aintín on a multiple choice exercise and they were both correct so I guess the space is optional! EDIT 2: ...it was a typo. Duolingo better fix it soon.
Dineen's 1904 Irish-English dictionary doesn't have an entry for uncail, but it doesn't have any mention of the word "uncle" in English at all. It also doesn't have any entry for aintín, but it does mention "aunt" in the definition of máthairín:
máthairín, g. id., pl. -idhe, m., a foster-mother; an aunt on the mother's side (O'N.); also máithrín