I research the Irish Family History Foundation's database. It contains 12,800,000 surviving rcds for abt 70% of Irish counties. Rcds are increasingly sparse B4 1840, rare B4 1800. Cos. Donegal and Derry were recently added to the database.
Note 1 - Without a KNOWN county, the chance of an ancestor's rcds NOT being in this database is AT LEAST 30%.
Note 2 - Only 1/2 of Co. Cork's rcds are in this database.
Note 3 - The RC church kept control over most parish rcds from Tipperary (north). These are ONLY available thru the Tipperary Family History Research center at a substantial fee (regardless of outcome).
Note 4 - Microfilms of parish church rcds are kept by the National Library of Ireland (NLI).
For Sam(uel) Millar M 1879 +/- 4 yrs, Co. Derry, I found 4 rcds: 1875, 1878, 1880, and 1881. None were in either Castledawson CI, or Castledawson PR parishes.
For Alice MaGee M ditto, I found 0 rcds. This infers the M rcds for Samuel are all false positives.
For Alice MaGee B 1857 +/- 4 yrs, with the father's forename unknown, I found 0 rcds.
For Samuel Millar B 1853 +/- 4 yrs, Co. Derry, with the father's forename unknown, I found 1 rcd, 1851. The father's forename was William.
For Samuel's siblings, with a father forenamed William, I find rcds for MOST of them, but rarely for the exact DOB you listed.
You listed lots of other names. Suggest you scour the database for their B/M/D rcds.
Google the Foundation's name to lead you to this database. Register, opt to be notified as rcds are added and browse. Pls post your search results to my genforum msg.