I would personally do the 2nd and 3rd week of May. Many of the later spring migrants have started moving in by the end of the 3rd week in May, and being here just before the middle of the month will allow you to capitalize on an early push (should it occur).
In terms of hotspots, hitting up the north shore of Lake Erie (Long Point, Rondeau and Point Pelee moving east to west) is your best bet for warblers, vireos and sparrows. Rarities show up every year, (including Kirtland's) and for a group from Ireland, the breeding plumages of the 20 or so species of warbler that are regularly seen on a per day basis will make the long plane trip well worth the headache. In good years these locations are phenomal, Point Pelee in particular if you can handle the large crowds (which actually make spotting things easier in a lot of cases). If you change your mind on shorebirds, try Hillman Marsh which has also yielded American White Pelicans, Yellow-crowned Night Heron and interesting gulls in years past. The marsh is a short drive from Point Pelee NP. A good day at Pelee/Hillman's Marsh will tally 90-100 or so total species.
As for the Kirtland's Warbler, the new nest site discovered in Petawawa is actually within the boundaries of a Military Base and is therefore off limits to the public (to the best of my knowledge)
Link to Warbler Article. While they do show up at Point Pelee or Rondeau in most years, the records are usually individuals and require being there at the right time.
Algonquin Provincial Park will offer a taste of Ontario's wilderness but will not get you the same numbers as the more southerly locations. Spring is also a good time to see Moose out on the water trying to escape the hordes of black flies (which may drive you to do the same at that time of year). The experiences from Pelee and Algonquin will basically be opposites. Pelee will be busy with people and hopefully birds and Algonquin will offer more seclusion and more chances for Canadian wildlife. (Rondeau and Long Point will not be as busy as Pelee if the large crowds are a turn off).
Closer to Brampton, the Hendrie Valley Trail (aka Valley Inn Rd) in Burlington as well as Samuel Smith Park in Etobicoke (Kipling Ave and Lakeshore Blvd. West) will be good warm ups to the previously mentioned sites. There are many others in the area as well that other people may mention but those are a good start.
As for a guide check out this discussion from a previous post:
Guide BooksHope that starts some more responses for you, good luck.
Iain
