Although it is a little unclear as to who invented the iris diaphragm first, there are many candidates that could be credited with the concept. A little searching throws up names such as; Robert Hooke, Joseph Nicéphore Niépce, and J.H. Brown.
What we do know, is that the first patent for an ‘improved’ device was awarded to C. C. Harrison (and Jos Schnitzer) in 1858. Up until then, it is suggested that previous designs were either fixed sized apertures, that needed swapping to obscure the desired amount of light. Or a flexible piece of rubber with a hole in it, which could be expanded by pushing a cone or tube against it.
You can read the patent here: https://patents.google.com/patent/US21470A/en