Hi Rob!
Another piece of the puzzle
...I fully expected the marriage certificate NOT to state Francis' father as John MULLEN/MULLIN. What you now have is the name of the man behind the McMILLAN part (I assume his father is given as Robert McMILLAN!?). Now just got to figure out who he was and where he fits in...ie: was he Ann's first husband? 2nd? Or was she ever married to him?
Some Questions: Does the date of the marriage McMILLAN / McMAHON match the date given on the birth certificate of Grandad John (which you posted first off on this topic)?
What occupation did it give for Robert and was he stated as deceased?
Who were the witness to that marriage?
Do Rose's parents on the certificate do tie up as John McMAHON and Rose McGRAW?
Did the marriage address say 22 Robertson or just Robertson - nevertheless, considering all the above, I'd say the odds on this marriage NOT being your Francis are extremley slim and the fact your Grandad John was later born on the same street is in this case, unlikely to be co-incidental.
Families did move often in those days - sometimes to a different house on the same street - and indeed sometimes to dodge the rent-man
In this case, I don't think it's a name-change per se; I think the answer will lie on where we can fit Robert in, and finding Francis birth certifitcate. One scenario is that he born to Ann and Robert McMILLAN but from a young age, the only father he ever knew was John MULLEN.
I also note - son John gave the information for Francis' death - it's very possible your Grandad John never ever knew of Robert McMILLAN or that Francis was ever anything else other than MULLIN.
So.....next step :
One one hand: In the McCART marriage, the one person who knew best was present - Ann. It may state she was nee FISHER formerley MULLEN and if she was ever married to McMILLAN, his name too.
On the other hand: Ann's death : we already know MULLIN and FISHER are indexed as other names she was know as - the full entry may or may not say McMILLAN aswell - if her son Francis or her husband McCART advised of the death it's possible, since we know they both must have known of McMILLAN....but then maybe neither was the informant.....
If it were me. I'd get both
but if I had to choose..maybe the marriage first?
This is a very intriguing topic
Looking forward to seeing what happens next!
Cheers
AMBLY