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Finding RC church records in Co. Donegal is very difficult pre-1850 and some parishes have records that start even later in the 1860s and 1870s. Civil registration for births in Ireland did not begin until 1864.
I searched the subscription site Roots Ireland for a baptismal record for Manus and did not find a record. Also, did not find a baptismal record or a civil birth record for Owen McCafferty. My guess is that Owen was born prior to 1864.
I then checked Griffiths Valuation data from 1857 for Co. Donegal. I located four Cornelius Cannon records which are shown below. They were located in four different civil parishes. Possibly the father of Manus was one of the four. I did not find a Martin McCafferty in the Griffiths.
I would suggest that you add a story for Owen and Manus to our XO Chronicles site. Possibly someone may recognize one or the other. https://www.irelandxo.com/ireland-xo/history-and-genealogy/ancestor-data...
I would also suggest a DNA test (autosomal) and from your matches you may be able to pinpoint where your Cannons or McCaffertys came from.
Let me know if you have questions.