Hello Louise, and greetings from the island of Ireland!
It's so exciting that you have found all this evidence, and we hope we can help you solve this mystery. :)
Before 1864 records were kept in the form of church and parish records, and while some of these have regrettably been lost over the years, all the surviving records have been sorted into county based
genealogical centres. Have you by any chance been in touch with the centre over in
County Down yet?
I wasn't able to find anything related to the expression 'Plunketts of the Shore', but it might have something to do with Strangford Lough or the area around the coast. Hopefully one of our genealogy experts might be able to shine a light on this puzzle!
If there is anything else you need, please don't hesitate to give us a shout.
Wishing you a wonderful weekend,
Melin