I'm not sure you will find any one person who has expertise in Canadian and Irish tax. You are probably looking at two specialists - one for Canada and one for Ireland. The Irish one should have some expertise in the structuring of an expat affairs coming home. You may want to talk to a second tier accounting firm with an office in Ireland and Canada. You can, of course, go for Big 4 but you may find the fees there on the high side.
I've worked in large firms (including Big 4) and medium firms myself.