"The children walk to us."
Translation:Siúlann na páistí chugainn.
It's just a difference of dialect. ionsar didn't even make its way into De Bhaldraithe's English Irish Dictionary, and it isn't included in the Grammar Database at teanglann.ie, so it's a preposition that isn't in wide use. It's not wrong, it's just regional.
(And I amn't Duolingo - I don't have any control over or direct access to the course material, I just get to delete the spam and irrelevant or misleading posts that crop up in the Sentence Discussions).