Hi Jack
Welcome to the IrelandXO Co Roscommon community!
THE SHIPWRECK
A tragedy of this nature would have been likely reported in the Freeman's Journal or local press in Ireland. Have you tried searching the British News Archives ?
GIVEN NAMES
"Martin" was an important name to this couple (probably the father of Pat or Margaret). Have you anything else to go on in terms of possible cousins that may have joined them. Any unusual names passing down? What naming patterns did Martin and Belle use with their children?
BTW "Isibéal" was a Gaelic name frequently interchanged with "Elizabeth" and even "Sibby".
ROSCOMMON-GALWAY BORDER
Further to Roger's guidance, here is a handy Griffith's Valuation map highlighting the areas where Gannon and Finneran were neighbours. (Seeing as Pat & Margaret married in Ireland sometime before 1847, the odds are very high that they lived in the same townland where sub-division practices were causing congestion. Or adjoining townlands).
You can ignore Elphin as the Church records there are excellent and something would have turned up in Roger's search.
Gannon + Finneran neighbours 1857 tells us pretty quickly that the civil parishes (along the Galway-Roscommon border) you should first explore are:
Athleague parish (Roscommon)
Killeroran parish (Galway)
Kilcloony parish (Galway)
Athleague / Killeroran seem to go hand in hand e.g. Finnerans of Easterfield baptise in ATHLEAGUE RC Co Roscommon but register these births in the civil parish of KILLERORAN Co Galway.
PARISH CROSS-CHECK
Have you tried searching the Pre-Famine (Tithe Applotments) and/or Post-Famine (Griffiths)? It might be interesting to see if you have any matches with the parishes maped above.
The DNA result may give you a better point of focus. If it points at all to north Roscommon, let me know and I can dig further.
Best of luck!