It is more than likely that you will never need to speak a word of Irish in Ireland. English is the lingua franca on the Emerald Isle, despite all efforts to resurrect interest in the Irish language. There are however a few word or phrases you will encounter more or less frequently. Some are in everyday use everywhere and some are used locally. And some may lead to confusion - from missing your bus to entering where you shouldn't.
Pronounced "guarda" and meaning "guard" or "guardian". Usually the short form of garda síochána, the "guardians of peace". This title was chosen for the Irish police and is used everywhere in the Republic. It is quite common to use the more English expression "the guards" in everyday speech. The police in Northern Ireland use the simple "police" as identification.
Not a tree but the Irish for "men" - you may find this as an identifier on toilet doors. Which can be quite confusing if not accompanied by a pictorial device or a translation.
3. MnáNot a misspelling of "man" but the Irish for "women". Again this is used as an identifier on toilet doors, mainly in the West of Ireland. The similarity of mná and "man" can lead to embarrassing situations.
4. Áth CliathPronounced "ah cli-a" and literally translated "ford of the hurdles" - the alternative name of Dublin (both names are Irish). Used on road signs, bus destination boards and similar. The preface baile (pronounced "bal-a") simply means "town", thus baile átha cliath is the City of Dublin as opposed to the county.
5. An LárLiterally "the middle" or "the center" and used on bus signs to denote the town center as destination.
6. SeirbhísPronounced "service" and meaning the same. The opposite is as seirbhís - "out of service".
7. SláintePronounced "slaan-sha" this literally translates as "health". It is used as a short toast between drinking companions, standing in for the more time-consuming "I drink to your health!"
8. SlánAgain the meaning is literally "healthy" or "safe", but this form (pronounced "slaan") is used to wish 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 "good bye".
9. Craic Agus CeolPronounced "crack agus col" (with a drawn-out o) this means "crack and music" and is the usual description of Irish nightlife. The word craic is not Irish - the English "crack" was simply re-written in an Irish form. The original meaning is "fun".
10. FáilteThe word (pronounced "fall-sha") means "welcome" and is used as a greeting 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".