That's the difference between a "slender n" and a "broad n". How obvious it sounds will depend on the speaker, the position in the sentence and the dialect (as in most things to do with broad and slender pronunciations, the difference tends to be stronger the further North you go)
The page will not let me reply directly (1) to the comment made elsewhere in this discussion by Knocksedan, "Combination of personal pronoun and possessive pronoun", so am replying here.
The combination is actually of a preposition and a personal pronoun -- thus: le+me=liom, le+tú=leat, le+sé=leis, etc.
(1) Moderators: why does that happen sometimes? Are replies rationed?
There are no active moderators for the Irish course (except the course contributors, who are unlikely to see your comment on a sentence discussion anyway). Because replies are indented, there is a limit to how many "generations" of replies you can have, to avoid the text becoming too narrow. At the 5th level, Duolingo doesn't generate the "Reply" button.
If you click on the link in Knocksedan's post, you'll see that the phrase "personal pronoun and possessive pronoun" is a direct quote from the source, Gramadach na Gaeilge. That source is talking about the preposition le, so it should probably say "combination with personal pronoun to create a possessive pronoun"
is linne an bia - "The food is ours"
ár bia - "our food"
Apart from the presence of a verb, "ours" is a possessive pronoun, "our" is a possessive adjective. Irish doesn't have possessive pronouns, which is why you need a whole sentence to demonstrate the equivalent construct - there isn't a single word translation for "mine" or "ours".