This is yet another song based on reoccurring premonitory, re-occurring dreams, pretty much like in 'Infinite Dreams'. The similarity between the two songs is striking when you examine the following lyrics:
"...scared to fall asleep again, In case the dream begins again"
"Scared to fall asleep and dream the dream again"
'Dreams Of Mirrors'
Steve Harris is known for having these dreams which, without being particularly premonitory, inspired many Iron Maiden songs ('The Number Of The Beast' comes to mind). He actually said himself in an interview with Johnny B. that the song was about the dark side of things, about people's thoughts and the suffering that may result from them, mostly at night.
Musically, it is a brilliant piece with a manic semi-instrumental part in the middle. The music matches the lyrics to perfection, giving the soft and disquieting tones of the one who is plagued by his dreams and who wonders about their meaning.