The Gaelic, or Goidelic, languages are a branch of the Insular Celtic languages. There are three Gaelic tongues, namely Irish, Scottish Gaelic, and Manx, which is historically spoken by the people of the Isle of Man. Each of these languages features a number of traditional farewells, including phrases for "Goodnight," "Goodbye," and "See you later."
In traditional Irish, "Slán go fóill" literally means "Health/Safety till later." It is most commonly translated into English as "See you later." "Slán" is used by itself for more permanent partings of the way and is translated as "Goodbye." "Slán leat", which means "Health/Safety with you", is usually used as a reply to "Slán." You can also use it if you're the one staying, with "Slán agat" being said by the one departing. If you're only parting for the night, use "Oíche mhaith" ("Goodnight").
As with Irish, there is more than one way to say "Goodbye" in Manx Gaelic. The most common phrases are "Slane lhiat," "Bannaght lhiat/lhium," and "Hee'm oo/shiu". "Oie vie" means "Goodnight." If you're not particularly fond of whoever you're bidding farewell to, you might tell them to "Go to the Devil," by saying "Immee gys yn Jouyll." Similarly, "Immee gys Niurin" means "Go to Hell." To tell someone that it's time for them to leave in a slightly more polite fashion, try "Immee royd," which can be translated as "Begone," "Depart," or "Run along."
The phrase for a formal goodbye in Scottish Gaelic is "Mar sin leibh." "Mar sin leat" is the more informal version. To be even more informal, try "Tìoraidh" or "Tìoraidh ma-tha," which mean "Cheerio" and "Cheerio then" respectively. Those who are only parting temporarily might say "Tìoraidh an-dràsta" ("Bye for now"), "Chì mi a-rithist thu" ("See you again"), or "Chì mi thu" ("See you"). To tell someone you're leaving, say "Feumaidh mi falbh," which means "I have to go."
Although these phrases may look difficult to pronounce at first glance, it is possible for non-native speakers to learn to speak all three Goidelic languages intelligibly. The best way to learn Gaelic pronunciations is to get coaching from a native speaker, be it a qualified tutor or just an individual who's passionate about their region's traditional tongue. Be aware that, although closely related, Irish, Manx, and Scottish Gaelic are effectively three separate languages and therefore each have their own unique pronunciation rules.