Partial Clones parish Church of Ireland records are indexed on the LDS FamilySearch site. You can search for Erwins/Irwins and McVickars there, although with a simple search, I don't see an Agnes Erwin born to parents John Erwin & Jane McVicker. The microfilm batch # for Clones is C70068-1.
What other details does Agnes' marriage record have, who did she marry, when and where? In what year(s) and locations do you find her in the Scotland census? What did she and her unknown husband name their children? (Parents' names are often given to children.)
All those details will help other researchers possibly assist you with research suggestions.
If Agnes' parents married between 1845 and her birth in 1850, and they were Protestant, you might find their marriage in civil registration indexes (also on-line at LDS FamilySearch and other sites). However, Co. Monaghan is currently only indexed by separate bride & groom names, and not full marriage records -- so you'd have to match up a John Erwin/Irwin's marriage date & place (with vol. & page in registry) with a Jane McVickers/Vicar.
If you'd like to share more of Agnes' biographical details, I'll try to help you further.