Your first step should be to obtain copies of the 1859 marriage and the 1865 birth in Ireland. These certificates should give you the townlands (addresses) where the events took place.
Tradition was to marry in the bride?s church (which should be on the certificate, unless it was a Registry Office marriage). That church may be the place to look for her baptism and that of any siblings.
You can order a photocopy of a certificate from GRO Roscommon for ?4. http://www.welfare.ie/en/Pages/Apply-for-Certificates.aspx
You have to download and print off the form. Then either post or fax it back to them. You can?t e-mail it to them. However if you want them to e-mail the cert to back to you, they will do that, so tick the relevant box.
Put the reference details on the form (anywhere). Don?t worry about leaving some boxes blank. As long as GRO have the location, name(s), year, quarter (where there is one), volume and page number they should find it.
You won?t get a birth certificate for Maggie if she was born 1862/63 as birth registration only started in Ireland on 1.1.1864. You might find her baptism though. The records may not be on-line and you may need to go to PRONI (the public record office) in Belfast to search them, if they still exist. (Some records have been lost over the years).
If you put copies of the 2 certificates on this board when you get them, I?ll try to help you trace back further and establish where the couple lived in Donegal.
Incidentally the town where Eliza & Kames? marriage was registered is Stranorlar (the site has it transcribed incorrectly).