I don't think this Dennis Buckley is your family. Findagrave says he was born in 1827 and his headstone is shared with a Hannah.
Dennis Buckley
Obituaries Johnstown Tribune
January 9, 1892, and January 11, 1892
He is probably this person in the 1889 City Directory. He is also on the 1880 census, and some marriage records for his children.
Buckley Daniel, steel worker, bds 200 Ebensburg Road.
Buckley Dennis, laborer, o h 200 Ebensburg Road, Pros. 5-5.
Buckley Maggie Miss. teacher, bds 200 Ebensburg Road.
Still, you need to ask why Rosanna and Michael Thomas Buckley came to Johnstown and not somewhere else. This Dennis might have been there before they were, I didn't look, and might be a relative. If so his information will help you locate what you need.
Overall - what I am driving at is that you have to get as much information as you can in the US records about anyone related or possibly related, if you stand a chance of finding Rosanna or Michael Timothy's parents in Ireland. Maybe you will find a sibling's information who provides the names. And it will help greatly to find mention of an exact place of birth or marriage in Ireland. Otherwise you are looking for a needle in a haystack in the Irish records.