Elizabeth,
You won’t get a birth certificate for Rose Ann if she was born around 1855. They weren’t introduced in Ireland till 1864. The only other option is a copy of her baptism, if you can locate it. Drung RC baptisms start in 1847, and they are on the rootsireland site. I checked them but did not find hers. I widened to firstly all Co. Cavan and then all Ireland but did not find her in either search.
What I did find in Drung’s records was Sarah Rose McMahon bapt 29.12.1853 to Bernard & Ann McMahon (mother’s maiden name not shown). The sponsors were Widow Brady & Thomas (no surname shown, it’s probably illegible). The family lived at Curavahan, so I’d say that’s Corravohy. So it’s the right family (especially with a Widow Brady as sponsor who is presumably Ann’s mother).
Also John McMahon of Corevahan (again that’ll be Corravohy) to same parents bapt on 30.1.1861. Sponsors Pat Hegarty and Anne Gilroy. If John isn't in the Scottish census records then he may have died. If that was before 1.1.1864 there won't be a record of it in Ireland. Obviously from 1864 onwards you can check Irish & Scottish death records. Likewise for Sarah Rose, unless it is possible that Sarah Rose is Rose Ann?
None of the other children you have named are in those rootsireland transcripts. Most RC parish records in Ireland are on-line free on the National Library site:
https://registers.nli.ie
However in this case there is a note on the website saying they don’t have Drung’s records. There’s a microfilm copy of them in the National Library in Dublin and another in PRONI (the public record office) in Belfast. But you have to go in person to view either set. Viewing them might reveal entries that have been missed in the transcription process or that some years entries are missing or too faded to be legible which might explain the missing records. That’s all I can suggest.
I also searched for Bernard & Ann’s marriage but did not find it. Drung’s marriage records only start in 1847, so if they married there before that, there won’t be a record.