"a hand and a head"
Translation:làmh agus ceann
They are (almost) completely interchangeable. When you see one, it’s always grammatically ok to use the other, as is is just a contracted unstressed version of agus.
There are some phrases and types of sentences when one is more common (and probably more natural to a native speaker) than the other, but then it’s just a stylistic choice. I wouldn’t worry about it too much when learning – it is the kind of nuance that is best absorbed naturally through contact with the language.
Here some suggest that is is more often used when listing things more closely related to each other and agus with more distant ones, but that’s no hard rule, and you can use both in much more contexts than just listing things.
EDIT: there is a discussion about it on Duolingo forums with more examples where one or the other would commonly be used.