Let me start off by saying that I'm not a Duolingo fan. I think it's a good concept, but when I worked through the entire course last year, it was only my prior knowledge of Irish that allowed me to maintain my sanity. There were just too many errors for it to be as a stand-alone learning tool, though it could possibly have some merit as an adjunct to another teaching method. I imagine that with a more widespread language, like Spanish, any errors and bugs would get ironed out quite rapidly, but that the number of Gaeilgeoirí is too small for enough experts to "patrol" the site.
I am not saying you shouldn't use Duolingo, only recommend that you don't rely on it alone, and that you be prepared to encounter some bizarre errors.