Hi Janice
Make sure, if you have not already done so, that you eliminate the database of Armagh Ancestry at
http://www.armagh.rootsireland.ie as a search of their index shows that they hold a baptismal record for Peter McCann in 1807 to parents John McCann and Bridget McCarraher and marriage records of 2 Margaret McCanns in County Armagh in 1838.
Although RootsIreland website is the largest online source of Irish church register transcripts, it must be emphasised that a failure to find relevant birth/marriage entries in this database doesn't mean that the events you are looking for didn't happen in Ireland. It simply means that they are not recorded in the database; for example, they may be recorded in a record source which doesn't survive for the time period of interest or in a source that has not been computerised.