Although there is no real equivelent of "hello" in gaidhlig, the english word is often considered acceptable when greeting someone. If you want to be more polite "Madain Math" (good morning) "Feasgar math" (good afternoon) and "Oidhche math" (good evening/night) are used. Though many dictionaries don't list it, the most common way to say goodbye is "Cheer-idh" (like you would say it in english, and it would be spelt "Tiree" except that that is allready a place name.