Hi Cathy,
Sorry it's taken an age to reply - I ended up getting ill twice with colds.

Anyways, about my previous post -- I think the link I sent you to the marriage cert is your grandad's brother, yep.
I also said that your grandad's dad, Joseph probably wasn't a sibling of John Hogg but I think I might be wrong now. Sorry if this is confusing but - John (my great grandad) had a few siblings, one of them was William but William was called William Joseph at birth, so I wonder if he maybe just went by Joseph through-out his life. Obviously I can't confirm this but I do remember poking about in some Irish records and thought of the possibility.
I finally got my DNA back and have been having a look through. I did match with someone who has Joseph Hogg, Margaret Hogg and their son Hugh (born 1902) - Hugh married Ruby Kate Nutting - daughter shown as Kitty Hogg.
I haven't found any matches in Downpatrick as of yet, but will def let you know if I do.
As for my gran, she was actually born in Scotland. She was the only one of her siblings to be born here. Then her mum died around a week after her birth, and I'm not sure what happened to her dad, John - whether he died in the war or went back to NI. But I do know from the 1921 census she was living with someone I assume was her uncle.
I decided to actually do a separate tree for the Hoggs and my findings - I also decided to put your grandad on there where I think he fits - unconfirmed obviously but it's just for a visual guide.
If you have an Ancestry account, you can view the tree here:
https://www.ancestry.co.uk/family-tree/tree/187670513/family/familyview?cfpid=432457388574