"They eat food there."
Translation:Itheann siad bia ansin.
Cad é an difríocht idir "ann" agus "ansin"
Nicole’s first post in your second link summed up the difference fairly well. Think of ansin as meaning ann sin — a demonstrative applied to ann.
Dinneen’s Foclóir Gaedhilge agus Béarla (1904) shows the spelling as annsin or annsoin, and defines it as “in that; then, there, thereupon”. Ó Dónaill divides ann into three categories — the first is summarized as “in existence”, the second is summarized as “place”, and the third isn’t summarized. Would you say that the “there” in “They eat food there.“ refers primarily to the food being in existence rather than primarily to eating in a particular place?
Siad is the subject pronoun, or "they" in English. Iad is the object pronoun, or "them" in English.
These just get a little confusing because word order between English and Irish doesn't line up. For example, when using the copula; in English, you would always say "They are people", whereas in Irish, it's more like you're saying "Are people them", so you use 'iad' ("Is daonna iad") there, for example, because unlike English it is functionally the object in the sentence.