The story is the most important part...then the rhyme...then the rest
I've always found that remembering a song is like remembering a joke...You need to know the basic story and the order it is told and then you learn the hook words (instead of a punch line)...these are the first words to each verse, the basic rhyming scheme/words, and any phrases that really grab you...the rest fills itself in with time, but in the meanwhile if O'Dougherty screams or any other 4 syllable named person it is not critical to singing "Back Home in Derry"