your version sounded very good to me and I also have played the Johnstons many times. I'm doing research as a job and one of my areas of research is memory and especially faulty memory. Your performance above is just a typical example for memory errors in normal adults (outside of a clinical context). Errors like this happen to everybody (not necessarily daily but more often than we wish for).
The typical pattern is that the errors are much more likely errors of omission (leaving out something) then of commission (inventing something which was not there). The only few inventions come from a kind of smoothing process (called: constructive memory) necessary to make the memory production consistent with expectation or story line. For instance, look at your line 1 in verse 4 (They thrashed me, and they steeped me, and they dried me in the kiln). It is nothing but a very creative 'invention' to make good for the lost verses in your recollection. The respective line from Colm (The next thing that they done to me they dried me in a kiln) would have made little sense in the context of your recollection. And that the confidence in memory productions is nearly always too high is so well known that rather the absence of overconfidence should make one worry.
You should praise your memory for the near perfect production of 4 verses (you have one or two words 'wrong' in each of the correct verses and not a single of these words matters).