I've always wanted to learn Irish (I've always loved Irish culture) but I also have a strong Scottish heritage, and want to learn Scots Gaelic too. I also love the language and music of the Brittany, and I'd even maybe dabble in Cornish. Is it a bad idea to learn all of these, albeit not at the same time, or should I just pick one Celtic language?
In my opinion, learning similar languages can be helpful as well as confusing. At the beginning, you might get the words mixed up a little bit, especially if your language skills are on more or less the same level for all the languages that you are learning, however as you get better, this will happen less and less. On the other hand, you will be able to notice words or rules that are similar, and in doing so will be able to grasp certain things quicker. Hope this helped. :)
If you have a passion for these languages, I think you should study them all. I would recommend choosing one language to focus on and learn well as this will open the doors to the others; being fluent in one language will make the others in that family more accessible.
Also, the Celtic languages are expereiencing a surge in popularity right now, so its a rather exciting time to be learning them; I bet it would be easy to find a group of people (on the internet or otherwise) with whom you can study.
Yeah, I'm definitely starting with Irish, but after that I think Scottish or Breton.
I personally pick the languages I like to learn, no matter how similar or different they are! What makes you think it might be a bad idea to learn both Scottish Gaelic and Irish?!