After 5 years of searching, I have at last found a few details about my paternal grandparents!
It's a bit complicated, please bear with me.
Starting from the top:
Margaret McDermott (married name) born 1833 Sligo, was widowed and remarried William Gallagher, born 1830 Sligo; a glazier in Carrick on Shannon in 1901.
Margaret's daughter Bridget McDermott, born 1873, had an illegitimate son, Thomas McDermott - born Boyle, Roscommon in 1889, (birth unfortunately not registered)
Bridget McDermott married (I believe in Carrick) to a man with surname Delaney and went on to have several more children.
Thomas McDermott married Mary Ann O'Hara, born 1886, Longford.
Thomas McDermott (who for many years was in Connaught Rangers) and Mary Ann spent all their married life in Carrick on Shannon, where the family was very well known, especially their eldest son Tommy, who drove a mule and cart through the town until his death around 1968.
Does any of this ring any bells with anyone?
Many thanks.