An Lon Dubh

Is “clann” in the accusative case? it would explain the n-, I guess…

It’s not in its accusative form, clainn, although you could still be right, as use of the accusitive was not necessary if the accusative is the object of a “do + verbal noun” construction in Classical Irish. Also prose (rather than poetry) did not follow the classical standard always, and often dropped the accusative form.

It might also be a genitive plural. We’d need to see the whole sentence.