Where It Is!
The study of Irezumi is the study of Ukiyo-e (images of the floating world), and any true devotee of the traditional Japanese tattoo should spend many hours looking at, appreciating, reading about, understanding and researching Ukiyo-e woodblock prints and paintings. Anyone interested in creating Irezumi him- or herself MUST study Ukiyo-e to have at least a minimal chance to grasp what is the reality of Irezumi. I've studied Ukiyo-e for many years and hopefully it has brought some degree of quality to my work.
A good place to start is at this exellent guide to Ukiyo-e websites. Hans Olof Johansson should have the Nobel Prize for his work of bringing Ukiyo-e to You, dear readers!