"Kirtsy actually added the the line 'so could anyone' in response to Shane's line 'I could have been someone' it didn't exist in the original lyrics, a touch of genius I think"

That'a not true - the line "so could anyone" is in the Pogues demo of the song with Shane & Cait O'Riordan, which pre-dates the version by Kirsty & Shane by several years.