Swearing in Irish.
You can't be considered fluent in any language until you can swear in it. And especially so in Irish. Cursing and swearing is very much part of the Irish cultural tapestry. And there is so much more to Irish swearing than 'póg mo thóin'.
Duolingo should seriously consider adding an Irish swearing bonus section. Here's a video that shows how beautiful and eloquent 'na rudai dona' can be.
I would warn you in advance that this video contains strong language and is not work safe. However, the chances of anyone understanding it are quite slim, so knock yourself out.
It's doubtful that Duolingo will ever implement this. There was a discussion not too long ago about allowing cussing on the forums, and they decided against it. The reason behind it was because of the number of younger kids who are using the site. If they don't allow it on the forums, it's unlikely they'll allow an actual lesson in it. However, if you head over to reddit.com/r/gaeilge you can see there's been a thread about cussing in Irish that gives examples.