"I am reading the historical newspaper."
Translation:Táim ag léamh an nuachtáin stairiúil.
And it's "nuachtáin" not "nuachtán" here because... ? (I'm light on the grammar, I know; this is really my entry into Irish.)
… because a verbal noun requires the genitive case in a following noun. (The adjective is also in genitive case, but stairiúil happens to be the same in the genitive case for masculine nouns.)