It's sort of like saying "health to you" or "good health" as a goodbye, as I recall
Did anyone else get only one word to study for this first section of common phrases? All I got was Slan EVERY time... XD
Slán looks like slain imagine: someone slays someone 'Goodbye! / slán!!!'
There is a difference. Depending on the emphasis that you want put in the statement, you could say:
is liomsa an leabhar seo ("this is MY book")
Is é seo mo leabharsa ("THIS is my book")
Seo (é) mo leabhar ("this is my book")