Vodafone and Meteor prepay customers can both roam on the Rogers network. You may have to select the network manually. Not sure about O2.
Voice calls are expensive and best kept for emergencies (Meteor are about 40cent per minute to receive calls or voice mails, ~€2.40 per minute to call Ireland, more to call Canada!). You might be able to get a prepay SIM in Canada. Or use a phone card with a landline, as suggested above.
SMS and MMS may not work reliably (Only SMS worked for me, only MMS worked for others I know!).