Presumably because "an" means "the". But it's a good question. In English, we would often leave off the word "the" in this phrase; in Irish, would they leave off the word "an" in the same contexts?
Why are so many sound files missing? Irish is hard enough when there IS someone to pronounce it for you; nearly impossible when there's not.
Creating, editing, and deploying recordings is an expensive task. (Many of the other language courses here can use speech synthesis software to inexpensively provide sound files for every exercise, but Irish isn’t supported by the speech synthesis software that Duolingo uses.)
I looked up "An lon" in and Irish-English dictionary which transkated this to "blackbird". Is that because I hadn't typed the accent mark over the o? Or, is it a secondary definition?