"A man, a woman, a girl and a boy."
Translation:Fear, bean, cailín agus buachaill.
After the singular definite article an, feminine nouns like bean are lenited.
bean is a feminine noun, so "a woman" is bean but "the woman" is an bhean.
fear, cailín and buachaill are all masculine nouns (yes, the Irish for "girl" is a masculine noun), so "a man, a girl and a boy" is fear, cailín agus buachaill and "the man, the girl and the boy" is an fear, an cailín agus an buachaill.
You don't need to use your English keyboard to enter fadas in your answers:
If you do want to actually enter the fadas from your keyboard (so that you can use them when you're not submitting them as part of an Answer to a Duolingo exercise), then the best procedure will depend on which English keyboard you are using - are you using Windows, a Mac or some other operating system, are you using a US layout (with # above the 3) or an Irish layout (with £ above the 3 key) or some other layout?