Hi tigerbelle,
You have a difficult task there!
Who was Denis Fitzgibbon? Was he your ex-husband's father?
You could search for the deaths of Denis and Mary on the English birth marriage and death indexes on a site such as Ancestry, if you have a subscription. Do you know if they were married in England, or Ireland? If in England, you could also search for their marriage on these indexes, and also the births of the children.
It is very hard to do research in Ireland over the Internet - there are not a lot of resources online. If you could find out a little more information about Denis and Mary, you could post on the Ireland board here and see if anyone can help you.
If you don't already have a subscription to a site such as Ancestry which has the bmd indexes, it would be worthwhile doing so, for the searches you need to make.
MarieC