I searched the Irish Family History online database at ROOTSIRELAND.IE. It has abt 20M B/M/D records for most Irish counties. Records before 1863 are increasingly sparse: almost non-existant before 1800.
For Marg(aret) Sherwood, B 1900 +/- 5 yrs, Co. Donegal, with the parents' names unknown, I found 1 B record in 1896. A view of the detailed record MAY show her parents' names.
For a W(illiam) and Marg(aret) marriage in 1919, somewhere in Ireland, with the parents' names unknown, I found 1 M record in Co. Donegal. A view of the detailed record MAY show both parents' names and sometimes other details.