"There are details at the newspaper." Where? in it's office? This is not good english and it should be translated as " ... in the newspaper..."
It's not good English because it's not a good translation of the given sentence. Tá rud éigin AG duine éigin is how you say "somebody has something" in Irish. So Tá sonraí ag an nuachtán means "the newspaper has data".
If you want to make a literal translation, then ag is "at", not "in*.
"There are details in the newspaper" would be tá sonraí sa nuachtán.