My brother and I visited the Kilkenny round-tower in the spring of 2010, and we were noticing the pattern of small gaps and out-cropping stones on the inner surface that appeared to form an upward-clockwise spiral. When we returned to the bottom of the tower, we asked the attendant about this.
He said that these are evidence of the original staircase, made of stone, and that "those last four steps" are the remnant. There are four irregular, dangerously slippery and worn steps right at the top, before the exit to the rooftop observation platform.
In recent years, all of the rest of the stairway was removed and plopped into the section below the entry, which is about three meters above ground. (That kind of destruction of ancient heritage would be impossible now.)
The attendant in fact decried the fact that permission to do anything is nearly impossible to get anymore. You can't bring in a shovel-full of gravel without extensive review, he said.
He also said that the reason the stairway spiraled clockwise upward was that the Irish people were left-handed back then. He didn't explain why left-handed people would build a staircase that way.