Translation:The mother makes the meal for them.
Maith thú! You are correct that the word is pronounced like "baylee" which is the nominative plural form. Since the definite article is "an" instead of "na" it appears that the dialect of the speaker is apathetic to the final vowel being a short "e" instead of a "long i" sound. Or else the speaker made a mistake.
On some progs broadcast from Donegal, I typically hear "daoine" pronounced like dee-nee. So there may be some of that influencing. Someday I'm going to get into the book Speaking Irish and start studying the dialect differences in more detail. Hope to God that they survive else the country will be that much poorer when they get standardized.
As I pointed out in the links in my answer, the Ulster pronunciation of béile is the same as in Connacht and Munster, and the current Duolingo speaker doesn't have any problem pronouncing béile in other exercises. She knows how to do it right, she just screwed up in this exercise, reading out the plural béilí after the singular an.
It's no screw-up. It seems to be just a variation in the way to say a word as is often heard from the same lips in English, e.g. idiot [ejit or idiot]; either [eether or eyether], etc. Béile features in at least nine recordings made by this speaker in five of which one hears "bayleh", and "baylee" in the other four.
This is pretty tricky, and it caught me by surprise, too. I looked at a chart and saw there are only two cases to beware of: 1) "díobh" means of/from them and 2) dóibh means to/for them.
Otherwise, the -ibh suffix always indicates 2nd person plural. And except for the two examples just discussed, a -u, -ú, or -eo suffix indicates the 3rd person plural.
I am bothered about the choice of déanann in the context of food preparation! ALL of my the Irish teachers ALWAYS insisted that the correct verb in this context is ullmhaigh! And if anyone ever made the mistake of using déin, we all got a very long explanation about why that was SO wrong!!!
From the entry for Déan in Ó Dónaill:
.3. (a) Prepare; bake, cook. Arán, tae, an dinnéar, féasta, a dhéanamh - "to make bread, tea, the dinner, a feast".
It also says that déan can be interpreted as:
(b) Eat, partake of. Do chuid a dhéanamh - "to take one’s meal"; "to feed oneself". Bricfeasta maith, suipéar mall, a dhéanamh - "to take a good breakfast, a late supper".
so context can be important, but this exercise is OK according to the FGB.