How could you get married in county jail?

If either of you are not in jail, then you would have to try to make arrangements with the sheriff, warden or whoever is in charge. Probably not a likely venue for your guests, though.

If I have misread this, and you wish to marry an inmate, it would be much wiser to wait until that person has fulfilled their time, stabilized their work and home life and has spent some time with you.