Go raibh míle maith agat - - Thank you very much.
The literal translation of this most common phrase is interesting and little known.
Remember Tá ... agat means you have. This phrase means
"May you have a thousand kindnesses"
Have a look at pages 822 and 823 of Ó Dónaill's
Irish-English Dictionary if you want to be
overwhelmed by all the meanings of maith.

Needless to say there are many, many other beautiful ways of saying
"thank you" in Irish besides this which I will list here from time to time
as they occur to me. Here are a few:
Mo sheacht ngrá thú! - - You are my seven loves!
Go méadaí Dia do stór! - - May God increase your fortune / wealth.
Nár laga Dia thú! - - May God never weaken you.
(If you know of others, a chara liom - - dear friend of mine,
please send them to me and I will include them here.)