Apparantly, Horslips based their song "Sideways To The Sun", on the album The Book Of Invasions, on the air Slán Cois Maigh (I assume "Sian" was a typo).  That's all I could find, but there is an air Sláan le Maigh; whether or not this is the same tune I don't know, but it was posted last year by Alison and Phillipa:  Tune add:Sla/n le Ma/igh, and there's a midi at Alan's site.

Malcolm