Well, thanks very much Shane!

So you think it's better to concentrate on the least common name and try to find the related surnames in the same area? That makes sense.
I'll tell you how I've been attempting to work this out using Griffiths, but I think I might have been approaching it the wrong way:
I searched for people with the same forenames and surnames as those in my tree, although I realise that the date of Griffiths doesn't match the dates these people lived in Ireland.
I found Dulton (difficult to interpret writing of surname on Scottish death certificate - could be any of other variations listed in my previous post):
County - Down
Baronly - Kinelarty
Union - Downpatrick
Parish - Magherahamlet
Townland - Dunbeg Upper
Place name - Dunbeg Upper
Place type - Townland
I found McVey:
Co - Down
Barony - Kinelarty
Union - Downpatrick
Parish - Kilmore
Townland - Drumnaconagher
Place Name Drumnaconagher
Place type - Townland
I found Lund
ay in:
Downpatrick St
Co - Down
parish - Kilmore
Lots of Lundys in Magherahamlet
There are no Ruggans or Roggans anywhere (the Scottish death certifcate had 2 "g's") but there are Rogans in:
Barony - Kinelarty
Union - Downpatrick
Parish - Magherahamlet
and also in Kilmore
I gave up there as I was SO confused. I didn't even attempt Murphy as I already knew that surname was very common. And I noticed that there were also Farrell's everywhere and there were lots of variations of the surname, perhaps it was originally O'Farrell.
I kindof understand the divisions:
County, Barony, Union, Parish etc (and there is a lot of info on the net about this) but how similar do the place names on the lists have to be before you can know whether or not these families lived close enough to eachother for their children to have met and paired off? Are there any really detailed maps which show all these divisions, so I see if these places are close to eachother?
It's like some kind of giant code designed to make my brain hurt. And at best all you end up with is an educated guess.

I do appreciate your help

- thanks again.