Hi and welcome to rootschat

So you have very little a 'Margaret' born in 1924, a 'Cath' who was Grandma, a possible connection to Roosky and a story about an orphanage, hotel in Dublin a divorce ( not in Ireland) and a couple of marriages and a death but no names, places or dates!
So a real mystery with nothing to start any research on and very few records available after 1911 to even look at.
So if I was faced with this I would be starting from her death/burial of Margaret (no surname) and searching first the death indexes assuming you know where she was living when she died, the looking for her marriage 'If' you find her in the death index and have a surname for her.
I may also search the 1911 Irish census for 'Cath' starting with Roosky Townland, Co. Roscommon, with an approx age and see what census shows, eg surname, names of Caths children, maybe a clue could include the name Margaret ( being a sibling of her mothers) ....... however Margaret and Cath are very common names and you could find yourself with lots of records especially with maybe a surname starting with M doesn't narrow it down as there are lots of Mc/Mac......
As until you find record proof there is no chance of finding anyone related and 'living people' are not something rootschat deal with, so be very careful about what if any info you post online about them.
Just as a side note, this mystery may take you many years to resolve ( or never) I had a great aunt who I knew very well, knew her name, researched her birth place/date, parents full names, siblings full names, her baptism record. I remember visiting her ( weekly) when I was small, so knew where she lived, knew the grave of her husband, know where and when she died and is buried...... she had no children, I could never find a marriage, was told he was Irish so I have over the years I have done the Irish research even thinking I found them ( turned out not them) but I had lots of information and eventually a rootschatter refereed me to a potential Scotland marriage and it turned out it was her, it would not have been a place I looked, found she was married, widowed, married again, no children but was a step mum to two plus found lots of things about her I would never have known and it took 20 years of research to get to that point and I had all the basics.....