Patrick,
I’d suggest you search all US records that might contain Mary Ellen’s surname to see if it is Callaghan, to be sure you have the correct family. And a DNA comparison with a living descendant of the 1933 marriage might be another way of checking.
The 1933 marriage tells us that John Joseph Guckion was a bachelor, a farmer; he lived in Gubnaveigh (spelling will vary, as with surnames). His father was John Guckion, a farmer. Going backwards from 1933, to the 1911 census, there’s a family in Gubnaveagh that seems to fit. Head of household was John Guckion and son John Joseph was aged 8 (so born c 1903). Does that age fit with your knowledge of John Joseph? What was his age in the US censuses and on his death certificate, for example?
I found a marriage for John Guckian to Mary Keaney in 1900. Copy attached. I have also attached the 1903 birth certificate for John Joseph (Just named John on the certificate, not John Joseph).
Family look likely to have attended St Mary’s Aughnasheelin, so worth checking there for graves etc.
I found the same family in the 1926 census for Gubnaveigh. Father John had died and his widow was head of family.
Griffiths Valuation for 1856 shows there were about 30 homes in Gubnaveigh then plus some plots of land and common grazing of the mountain top. Andrew and Micheal Guckran (likely a spelling error for Guckian) are shown on plots 30 & 31 which totalled just under 28 acres. Those were the only Guckian properties in the townland, so probably where your family lived. Looking at the properties today on a satellite view (which is available on the Griffiths Valuation site) the buildings shown on the maps in the 1850s are gone, but there is a modern house nearby. So you could knock on the door and see who lives there and what they know of your family. It’s up an unnamed dead end lane, off another unnamed lane, off Mullnasillagh. 2 or 3 miles from Aughnasheelin. So you’ll need to polish your navigational skills to find it.
https://www.askaboutireland.ie/griffith-valuation/index.xml?action=nameSearch
Be aware that Gubnaveigh is some distance north of Mohill (perhaps 20 miles).
There are several trees on Ancestry which list this family. One (“Guckian family tree” owner Ann Fanning) records that a Francis Guckian died at Gubnaveigh on 25.5.1957, so there were still family in the townland in the 1950s. He was apparently a brother to John Guckian who married Mary Keaney in 1900.