I looked here: http://www.irishgenealogy.ie/
Try the search facility there. You'll see that Daniel and Mary were married in Leitrim in 1869.
If you want to find this record yourself, you could first look at the 1869 marriage index to find what volume (there were 20 each year) of the marriage records holds the record for Daniel and Mary. There would be a film for that volume.
Or you could ask the Leitrim Genealogy Centre [email@example.com] to look it up for you. They would charge you 20 Euros.