Thompson
To my ear , Hi is short for hi ya , which is a blending of the three words “how are ya”
( conas ta tu )
You will still hear older people when they meet in Dublin asking “how’re ya ?“or “how are ya ?“ . Over the past twenty years ,this how ya/hiya ? has been gradually replaced with "alright ? ” - the common British greeting.   

I often heard YO being used as a greeting last time I was in the US