While it is not essential to learn Irish before you visit Ireland, it is a good idea to have a few words in your pocket in order to avoid the potential for confusion and embarrassment - from missing your bus to entering where you shouldn't. So let us start on a journey through the basic words every visitor to Ireland should know.
More than likely the very first Irish word you will hear (especially if you are flying on Aer Lingus), "Fáilte", pronounced something like "fall-sha", simply means "welcome." You will hear the word widely used both as a greeting, see it on signs welcoming you into a new town, or to denote tourism activities - the Irish tourism industry uses the moniker "Fáilte Ireland". The very popular phrase céad mile fáilte ("kad meel-a fall-sha") translates as "a hundred thousand welcomes".
This will be more than likely your first destination in Ireland, and might even refer to the airport you are flying into.
Pronounced "ah cli-a" and literally translated "ford of the hurdles" - this is the alternative name for Dublin (both names are Irish, by the way). Used on road signs, bus destination boards and similar, all over the country (except in Northern Ireland, where a plain "Dublin" is used). The preface baile (pronounced "bal-a") simply means "town", thus baile átha cliath is the City of Dublin as opposed to the county.
Getting from the airport to the city center is no problem, just hop on a bus that takes you to "An Lár", a useful Irish word to know if you want to get downtown.
Literally "the middle" or "the center", and frequently used on bus signs to denote the town center as a general destination. While it is a good word to know if you plan to travel around Ireland, be warned that it is an imprecise description of where the bus might be headed. The definition of center in Dublin, for example, covers a wide area, roughly between St. Stephen's Green and O'Connell Street. It is far easier in smaller cities like Galway, where you'll be more than likely be dropped off near Eyre Square - the heart of the city.
Bus not going anywhere? In that case the destination board might have read something different from "An Lár", maybe the seemingly similar "As Seirbhís"?
Seirbhís is pronounced "service", and it means the same. The opposite, however, is as seirbhís - "out of service". Frequently seen on buses, as these tend to travel empty from or to the depot (in other countries, routes actually start and end at or near the depot; in Dublin especially they tend to terminate or kick off as far away as possible).
Time to say your good-byes? Well, then do it in Irish fashion as well!
Similar to "sláinte", the meaning of "slán" again is literally "healthy" or "safe". But this short form (pronounced "slaan"), it is used to wish somebody a safe journey and healthy return. The extended slán abhaile (" slaan aval") is used by the host and means "safe homewards". Other forms are slan agat and slan leat, all meaning "goodbye".