I was wondering some of the same things a few years ago and found there to be quite a lack of good info so hopefully I can share some of what I have learned over the past few years working and talking with forensic psychiatrists and also reading.
Forensic psychiatry is just where law and psychiatry interact. That can be a number of different things but typically for the criminal side, you mostly see competency to stand trial, mitigation of penalty, and ngri (not guilty by reason of insanity) evaluations and on the civil side, you probably see more competency and disability evaluations.
Q: "is there a strong job market for these docs?"
A: Psychiatry in general is experiencing shortages. You especially see it in subspecialty areas such as geriatric, child, forensic, and addiction also. So there is a good job market but it also depends what you want to do. If you want a salaried position at a state hospital or in the penal system, you are in great demand. If you are trying to strike out in private practice, you need to do a good bit of business marketing for lawyers, companies, etc.
Q: "how difficult is it to get a fellowship in forensic psych?"
A: There are about 40 or so fellowships to date, as far as I know. But remember there a lot fewer applicants applying than for the general residency positions. You should have no problem get a spot.
Q: "do they command a quality salary?"
A: Money is also variable and depends very much on how good you are, how good you market yourself, and what you do. Overall, forensic psychiatrists make more than general psychiatrists, but remember, there is more training involved. In salaried positions, the pay is slightly higher than general psychiatry. In private practice, the rates can be better but remember that the hourly rates of some are not sustained on a full 40 hour week usually and the really high rates you hear about are by a handful of forensic psychiatrists at the top of the game.
I wanted to make a few comments also. From what I have been told and learned, the best forensic psychiatrists are ones who love the work and like being around lawyers and enjoy thinking in that way. The worst ones are the ones that go into it searching for money or any "easy way out." Forensic psychiatry can be much more stressful. Everything you say, write, and do can and will come under scrutiny in cross examination, depositions, and while you are on the stand. There is a level of precision and attention to legal concepts in this work. Writing is a critical skill. Some feel detached from medicine and do not like being in a sometimes, adversarial role versus a normal therapeutic doctor-patient relationship. Others hate having there autonomy, intelligence, or competence challenged by opposing lawyers. These things being said, if you like the law and being around lawyers, forensic psychiatry is a very interesting field. If you like the field also, you must really know your clinical psychiatry well too and in most places, need to be practicing at least somewhat. One last thing, if you are concerned about dangerousness (from your earlier posts), many times you will work with very dangerous individuals and seasoned criminals. Some find the risks to be a little higher, if that bothers you.
Hope that helps - I was in your shoes before, frustrated at the lack of info.