Are PhD supervisors responsible for informing students about legal issues pertaining to research? After an extreme amount of effort, I am now in possession of a very large dataset of human experiments which is required for my research. However, after speaking with some of my colleagues about writing papers about it, I was told that I need to have had the IRB decide what level of consent is necessary and then had each participant sign the consent form specified by IRB... or else I can't distribute the data... to anyone (yes, I know this is country-specific, but most countries require at at least some legally-valid form of consent).

I hadn't thought about this because I was told by my supervisor to collect the data... and so I did. Now, after asking my supervisor about the ability to legally distribute the data, he agrees with my colleagues in that I need to have had them sign a consent form... but if he told me to collect the data, then methinks he could have been prudent enough to say "oh, and before you go persuade 50+ random people to participate in an experiment, don't forget to have them sign this form or else you can't share the data with anyone at all".

Are PhD supervisors responsible for telling their students about the legal requirements of their work? If not, is it expected that all "learning" done while a PhD student be from making horrible mistakes? Should I basically treat my supervisor not as a boss but as just some random person who might not actually know what I should do?
Good research practice (which I would argue is not country specific), requires an independent ethical review of any research that involves human subjects. A university can make blanket determinations that certain types of research do not require individual review (e.g., interviews).

It is generally considered academic/research misconduct to not have proper ethical review and follow the recommended guidelines. Failure to obtain informed consent, when necessary, definitely falls under research misconduct. In the US and UK, it is the responsibility of the supervisor to make sure that the proper procedures are being followed.

In your case, your supervisor should have told you to get informed consent. The problem, however, sounds way bigger then you just not being able to use the data. The problem could be as large as PhD students not being properly trained regarding human subject research and faculty not following good research practice.

You need to work this out with your supervisor quickly. It should eventually escalate to your department chair and the IRB/ethics board. Not getting informed consent, when needed, is a huge deal.