This is a de facto Irish-English word, an affirmative exclamation. It consists of a rapid insuck of air, and sounds a bit like a gasp. Sometimes it is pronounced a bit like "yeah;" but not always. Most common amongst women.
Irish men will often greet with a sideways nod when passing on the street.
Use of your given name
If an Irish person knows your first name, he or she will probably call you that, even without introduction.
"Eyes to heaven"
An uptoss of the chin accompanied by a slight click from the tip of the tongue and a "you know y'rself" rolling of the eyes upward.... This expresses acknowledgement of the sublime and ridiculous nature of reality.