Many ages and dates of birth on official documents in Ireland were just a guess.
Alexander Irvine was born in 1863 in Antrim town and became a Minister living in the US. This extract from his book “The Chimney Corner revisited” perhaps explains why people often had to guess their ages:
“My mother kept a mental record of the twelve births. None of us ever knew, or cared to know, when we were born. When I heard of anybody in the more fortunate class celebrating a birthday I considered it a foolish imitation of the Queen’s birthday, which rankled in our little minds with 25th December or 12th July. In manhood there were times when I had to prove I was born somewhere, somewhen, and then it was that I discovered that I also had a birthday. The clerk of the parish informed me.”
I have attached a letter which I found in parish records in PRONI (the public record office) in Belfast from someone in Pettigoe, Co. Donegal in 1908 writing to his Minister, asking for proof of age (ie a baptismal certificate). All he knew was that he was between “70 and 78 years of age.” He clearly had only the vaguest idea and couldn’t narrow it down to within 9 years. (The likely reason for the letter was that the old age pension was being introduced in 1909 for people aged 70 and over. Documentary proof of age was required. Thus, probably for the first time in his life, establishing his age accurately became relevant to him).
Regarding the size of the population, a townland was anything between 1 acre and 2000, though the average was probably 300 or 4000 acres. If we look at Porthall in Griffiths there were about 28 houses listed. In the 1901 census there were 41 houses and a total population of 169 people. So with those small numbers, if we find someone with the name you are looking for, at the time we know is right, there’s a fairly strong chance it’s your family. (We know Joseph lived in Porthall in 1851 and his father was named William. In 1858 in Griffiths there were 28 houses in Porthall and just one McCain, who was named William. Statistically it's likely to be your family. Can't prove it for certain though without other records).
http://www.census.nationalarchives.ie/pages/1901/Donegal/Clonleigh_North/Porthall/
Interestingly, looking at Porthall I see 1 McKean household there, plus another McKean lodging with another family nearby:
http://www.census.nationalarchives.ie/pages/1901/Donegal/Clonleigh_North/Porthall/1195725/
http://www.census.nationalarchives.ie/pages/1901/Donegal/Clonleigh_North/Porthall/1195727/
So they could be related to your family, though the farmer was Presbyterian whereas your family was Church of Ireland so they may not be connected.
McKean family in Porthall in 1911:
http://www.census.nationalarchives.ie/pages/1901/Donegal/Clonleigh_North/Porthall/1195727/
I researched them a little. William McKean married Isabella Fyffe in 2nd Strabane Presbyterian on 20.1.1909. (Tradition was to marry in the bride’s church). William’s father was James McKean.
What looks to be William’s birth was on 23.6.1871 in Rateen, Taughboyne to James McKean & Esther Hamilton. (Thomas McCane was born in the same place 1st March 1874._
James McCain married Esther Hamilton 19th March 1868 at 2nd Ballylennon Presbyterian. He was living in Rateen and his father was also James. So it looks as though this McKean family did not arrive in Porthall until some time after 1874 (when Thomas was born). So they weren’t there in the 1850s. So possibly not connected to your family.
There’s a Catherine McKean death in 1870, aged 62, registered in Strabane. That could be William McKean’s wife (maiden name McCrudden). The age seems about right. That death is not on-line free yet. You would need to buy a copy from GRO Roscommon. I can explain how to do that if you are interested in seeing it.