In all of my writings you would have seen that I feel that the study of language is an important part of studying the culture of your hearth religion and so for years now I have been studying and re-studying a multitude of resources to learn Irish. One of these resources is Duolingo.
Duolingo is an app and website that teaches different languages including Irish and Welsh. I'm not sure about how good the Welsh lessons are but I can speak to how good the Irish lessons are...
I've used Duolingo on and off almost from the beginning of the Irish lessons when it was still in Beta mode. However, this time I used it for a little over 2 months straight. This usage has confirmed what I felt about this app/website from the beginning.
There isn't a lot of grammar introduced with the lessons. Sometimes grammar structures are introduced without telling you why it was the way it was and it becomes confusing. Not all the phrases or words have audio files. Also, the sentences the app/website uses are sometimes very funny like the one about the dog reading or even paying for the cat. Still it does get you used to the language a little bit.
If you are seriously thinking of studying Irish, don't just use Duolingo, it will not be enough and it sometimes can cause harm; do supplement it with something like the book series Buntús Cainte and/or Progress in Irish. Duolingo is a good tool to learn vocabulary but be prepared like I said for the curve ball words with unexplained grammar structures.