The bacteria in yogurt usually destroys almost all the lactose, so even lactose intolerant people generally can eat some amount of yogurt. If you are allergic to milk and milk products, then you need to stay away, but if you are only mild to moderately lactose intolerant, then you can try some.
Yogurt is good for you for the calcium (which is benefcial to the diget tract becaue it can help soothe an inflamed/irritated digestive tract. Also it has probiotics (good bacteria) which are good for anybody.
And not all IBSers are lactose intolerant; I have no problems consuming dairy.