Craic

http://www.urbandictionary.com/define.php?term=craic:

1. Craic

Irish word for fun/enjoyment that has been brought into the English language. usu. when mixed with alcohol and/or music.
'Bhi craic agus ceol againn' : We had fun and music.
Fun doesn't really cut it though. General banter, good times had by all.
Also, a person who is good fun/great company.

It was great craic.
She's great craic when she gets going.
He's great craic when he has a few pints on him.
What's the craic?
How's the craic?
The craic was mighty.

Note: Very tricky to get away with saying this in the US without getting strange looks [from] police officers.