The notion of IRB is actually international and covers many different situations. I don't know all of the variations that are used, especially by large companies that conduct medical and pharmaceutical research. But I doubt that there are very many people in the world who are specifically hired as IRB (or equivalent) members. Hence, I doubt that there is really a career path for this.
However, if you are interested in doing it, I think there are two ways that you can work towards being selected for it. The first is to become a prominent scientist in your field of interest, perhaps psychology. The other is to become a prominent philosopher of ethics. Then your skills and opinions would be seen as valuable to those making up an IRB.
That isn't to say that you can't be paid as an IRB member. Some people might be hired as (part-time) consultants to serve on an IRB or to give advice in some situations. Faculty members who serve (part-time) on an IRB, can get released from other duties and hence paid indirectly. Or you might even be a full-time member in a large institution, but likely for a limited period, returning afterwards to normal research.
The requirements in the US, require that a team consist of at least five members, with at least one scientist and at least one non-scientist. A full time career would mean that you aren't really a scientist anymore, so that you wouldn't qualify for the scientist slot if it were full time over a long period.
But if you want to do this, learn a lot about science (and practice it) and learn a lot about ethics (and practice that too).
I'll also note that one of the problems with being a full time IRB member in a for-profit organization that needs to monetize its research is that that there can be a lot of pressure to allow things that should not be done. Standing in the way of what some vice-president really, really, wants to do doesn't make for a secure future.
But one of the most valuable things an IRB can do is to look at a problematic proposal, decide what is fundamental in the research goals, and then suggest an ethical way to meet those goals. That sort of thing takes a lot of experience and sound judgement.