What may be George Johnston’s marriage was registered in Cootehill registration district 1857, Volume 4, page 399. (Wife’s maiden name Jordan). If you get a copy of that certificate it may give you information with which to trace the family further.
You can order a photocopy of the certificate from GRO Roscommon for €4 (euros). 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.
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.