Hi John,According to the notes in "The Poolbeg Book of Irish Ballads"(Sean Mcmahon). It is about "Innocent flower, Lilium bulbiferum, which has become a political symbol.A rougher version may be heard in July throughout the marching season."
If I remember rightly the orange lilies bloomed around the 12th of July. (but then, so they should.. it's summer.)
Slainte
alison