This technically isn't in Ontario, but it's really, really close by.

http://www.buffaloirish.com/irishfestival.htm
The Buffalo Irish Festival is August 28-30.
There will be a good selection of traditional and contemporary Irish music, decent priced Guinness and Harp for those who drink, and plenty of seating and shade for everyone.

And purely by coincidence, it's the weekend that I return from Georgia.
I know that the SOKS have already got Fergus and Sir William's visit in August, but I thought I would toss it out there if anyone's interested.