Cork slang. Depending on the context means "Go on ahead"/"keep going" or "well spotted" Very much a northside version, so uttering this phrase while wearing a tracksuit adds gravity to your statement.
Chap one: "I say, would you care to indulge in a beverage with me?"
Chap two: "Nah, boy. I'm going out with the oul doll. Ball on."
Chap one: "Oh dear. There appears to be a group of drunken lingerie models in the corner who are attracted to men with buzzcuts and homemade indian ink tattoos over there!"
Chap two: "Wha?!? BALL ON!!!"