Scotland's People is the place to get Scottish certificates- this tells you what details are on marriage certificates:
http://www.scotlandspeople.gov.uk/Content/Help/index.aspx?r=554&403
We might be able to help locate information from the Irish birth certificate so if you'd like help with that let us know what details you already have and we'll see what we can find or at least tell you the best way to get that certificate (there are different options depending on where in Ireland).
Thanks again, for being so helpful Aghadowey.
I haven't had any time to pursue this since my last reply. It's a real shame to have to do this research actually because one of my aunts traced the whole family tree but it was before the days of computers and her record of it kept on an old piece of wallpaper got accidentally thrown out when she died a few years ago.
All I knew was that my grandfather Patrick Plunkett was born in Ireland and was taken to Scotland as a baby and that he came from somewhere very near the border, which after partition turned out to be in just north of the border. I know he died in 1964 in Coventry and that his age was listed as 83.
After searching the Irishgenealogy site the only place listed with Plunketts very near the border was Clones. My sister later remembered that this was the place my Aunt mentioned. In fact my sister once visited the area and tried to located the actual village but without more precise info wasn't able to find it.
The entry I found was the following:
Birth of PATRICK PLUNKETT in 1881 from
http://civilrecords.irishgenealogy.ie/Group Registration ID 10842656
SR District/Reg Area Clones
Sex N/R
Mother's Birth Surname N/R
1881 fits with the death in 1964 at age 83, so I'm confident it is him. I don't know his father's first name or when he died but the Plunketts aren't too numerous in Clones, so after perusing the records I strongly suspect that his father was John Plunkett married in 1866 (my father's oldest brother was called John but it's another common name, so I'm not sure how meaningful that is).
Patrick's father would have gone to Scotland with Patrick and any of his siblings but my sister told me that she thought he had returned to Ireland shortly before his death. However, I found no record of a death of John Plunkett in that part of Ireland up to 1940. That would perhaps fit with him being in Scotland (if indeed he was Patrick's father) and perhaps she mis-remembered and he was merely buried back in Ireland rather than having died there. We have no idea what year he died.
Again guessing from dates, a possible father to John would be James Plunkett who died in 1877. Unlike Patrick's mother's name, none of the above is essential to the passport issue, but it would be nice to know, so any info you can discover would be gratefully received.
Sinann mentioned the possibility of getting the certs from GRO in the North. Is that because that's cheaper or because as Clones is now in County Fermanagh, that's what one has to do?