Each language has its own way of thinking, so you can't expect them all to be consistent with English. In Irish, it seems that:
If you have something in your possession, it is "at" you (agam, agat, etc)
If you own it, it is "with" you (liomsa, leatsa, etc)
It you want or need it, but don't already have it, it is "away" from you (uaim, uait, etc)
Like Gp6am said don't compare the languages literally. That will totally mix you up. There is also many different ways to say it correctly, but that doesn't mean that Duolingo will have all the variants.
In Conamara we prefer to say "ag iarraidh" for "want".
An bhfuil tú ag iarraidh..... ?
And "teastaíonn" for "need".
Teastaíonn ... uaim.
I'm a beginner myself, so this may be totally wrong, but someone with a bit more knowledge told me that the pronunciation of "bh" tends to vary with its position in the word: a "w" if in the middle of a word (ex: "leabhar"), a "v" if at the beginning or end of the word (ex: "bhean," "agaibh"). However, in "bhfuil" where it's being used to soften an "f" next to a "u," the "v" sound has shifted towards a "w," often being completely absorbed into the "w" sound.
There seem to be many variations in the pronunciation of Irish words, depending on what part of Ireland the speaker comes from (Connacht, Munster, Ulster, Dublin, Donegal, etc.). That's why you may hear the "duit" in " Dia duit" pronounced "dit" or "gwit." Don't worry about it too much. Just get used to recognizing the variations when you hear them.
I often wondered why Irish didn't become the actual first language of the majority of people on the island of Ireland and I think the many variations and complexity of it limited its use to an enthusiastic minority despite being taught in schools and promoted by Government. Perhaps the powers that be should come up with a standardised simplified version and more rational pronunciation and spelling to further that end. After all its mainly about identity and I feel a country which mainly uses the language of another country is not fully independent in the way France for example is