The scenes with giants and little people in the same frame were often done entirely in-camera, by positioning the giant actor in the foreground and the little actor much further back to appear smaller in a 2-D picture. The shots were all story-boarded and designed in advance with mathematical precision regarding distances, angles, eye-lines, and what lens to use.
Actually, Zap, they didn't use that method all that often; the giants were just too large in comparison and the soundstage only went back so far. The little people had to be small enough to fit in a giant hand. They mostly used split screens, mattes and rear projection. The actors often said they never met their giant co-stars.
When Irwin DID use forced perspective, it was on shows like Voyage and Lost in Space when a giant creature (or giant Dr. Smith) wasn't as large. The Voyage episode "Leviathan" is a perfect example of nearly flawlessly executed forced perspective. The camera even pans across the set to reveal the giant, which could never have been done with mattes and split screens, at least not with the time and money they had.