Firstborn Son or Eldest Known Surviving Son?

It was virtually universal in every class and creed in Ireland for the firstborn son to be given the Christian name of his paternal grandfather. One can presume this with a degree of genealogical surety — provided one knows the name of the firstborn son, which, in an era of high infant mortality, was not necessarily the name of the eldest surviving son.
– Rosemary ffolliott, “Irish Naming Practices before the Famine”*

This is an obvious point, succinctly stated by Rosemary ffolliott in the passage cited above. And yet, I’ve seen enough people jump to hasty conclusions based on the name of the eldest known son that I think it bears repeating: if you don’t have the complete parish records for a given family (with all of their children’s baptismal records all lined up nicely in a chronological row), then you cannot assume that the name of their eldest known son gives you the name of his paternal grandfather.

So, for example, my great-great-great-grandparents James Moran and Margaret Jamieson emigrated from Ireland to Canada about 1820 (but possibly as early as 1818), and can be found in Huntley township (Carleton Co., Ontario) by 1821. Various Canadian records (especially census returns and Roman Catholic parish registers) allow me to reconstruct a family of three sons (Thomas, James, and Alexander ["Sandy"] Michael) and seven daughters (Marcella, Mary, Margaret, Julia, Elizabeth, Anna, and Henrietta), with the eldest known son, Thomas, born at Huntley about 1822. Can I therefore conclude that James the Irish emigrant was the son of a Thomas Moran back in Ireland? 

I cannot arrive at any such conclusion, much as I’d like to. I do not have the complete parish record for this family, and I doubt I ever will. 
The first baptismal record I have found for this family is for their eighth known child, Elizabeth, apparently born 2 May 1833, and baptized 25 September 1833 (the record is in the parish register for Notre Dame Basilica, Ottawa, though she would have been baptized in Huntley by a missionary priest), with John Lahey (probably also my ancestor, through another branch) and Mary Lahey serving as sponsors/godparents. Now, of course I didn’t just invent the first seven children, for whom I lack baptismal (which is to say, prior to civil registration, birth) records. They are in the 1851 census in their parents’ household, and some of them go on to marry as daughters of James Moran and Margaret Jamieson (I have no marriage records for the sons, and as far as I know, only one son, Alexander Michael, ever married, but I’ve yet to find the record for his marriage to Mary Leavy, daughter of John Leavy and Jane Byrne),** and then there are some burial records, and so on. But the point is, the 1820s are a bit murky (what with the lack of baptismal records and the lack of personal census data), and there may have been a child or two who was born and died with no known surviving records, and I only know about the children who survived to adulthood and made it onto the information grid.
So to repeat the obvious (but in a detail-oriented pursuit like genealogy, the obvious sometimes bears repeating): the eldest known surviving son is not necessarily the firstborn son in a given family.
*The Irish Ancestor, vol. XVIII, no. 1 (1986), p. 1. 
**Thomas (born about 1822) never married, and died at Huntley 21 January 1892 (buried 23 January 1892 at St. Michael’s RC Cemetery, Corkery). James (born about 1824 at Huntley) just barely made it into the 1851 census: he was enumerated as part of the household, but the census taker noted that he had recently died of “collara” (cholera) (note: the first time I looked up the Morans in the 1851 census of Huntley, I completely overlooked the recorded death of son James: always look in the margins and under all columns [yes, even at that stuff that looks all boring and technical], you never know what you might find).