My mother was an O'Neill and she had two brothers, still both living, who are O'Neills. My mother's father, my grandfather, was John Graham O'Neill (although throughout his adult life he preferred to be called Graham and signed his name J. Graham O'Neill). That is the easy part, the part that I know from personal experience.
Getting beyond my grandfather to my O'Neill great grandparents was not too much of a challenge thanks initially to the O'Neill family cemetery plot in Mount Hope Cemetery in Toronto, Ontario. The photo below shows the headstone from the O'Neill family plot. Clearly marked on the stone are the names of the four adults and one infant buried in the plot. Missing from the headstone is my grandfather's name who is also buried in the plot.
William Emmett O'Neill was the first to be buried in the family grave when he died in 1924. He was joined in the grave in 1927 when his grandson, John William O'Neill, the infant son of John Graham O'Neill and Gertrude O'Neill (nee Foley), died. William's wife Margaret O'Neill (nee Graham) died at 189 Pickering Street in 1937 (creepily in the very bedroom in which I was conceived according to what my mother told me) and was interred with her husband and grandson.
When William Emmett O'Neill and Margaret Graham married on June 4, 1894 at St. Mary's Church in Toronto, Ontario, they would not have been considered to be young people. William was listed as being 42 years old and Margaret was 39 years of age. Their marriage registration indicates that both were Roman Catholic and that they were both born in Canada. Unfortunately, this means that they were born prior to the commencement of civil registration in 1869. Remarkably, despite their ages, William and Margaret had three children - a boy (my grandfather) and two girls, Kathleen who would enter the convent and become Sister St. Edwin, and Avila who never married.
The records about this family, though at times difficult to find, paint a picture of a middle class family that seemed to maintain a comfortable but not luxurious lifestyle. In 1911 for example, the census shows the family living at 400 Margueretta Street in Toronto's west end. In that census, respondents were asked to report on their annual earnings in 1910. William reported that he had earnings of $500 from his work as an insurance agent. To gauge the current equivalent value of $500 in 1910 today, I used a number of online calculators. The buying power of $500 in 1910 is about $11,500 today, hardly a wealthy annual salary. The better comparison however was able to be drawn from a Statistics Canada report that shows the average salaries or earnings for production workers and supervisory and office workers. William's earnings in 1910 were slightly above the average for production workers but was about half the annual average earnings for supervisory and office workers.
William listed his parents as being John O'Neill and Mary Murphy on his marriage registration. Just as the headstone indicates at his grave, William also consistently listed his birth date as February 26, 1849 in census records. For his marriage registration however, when he indicated his age to be 42, the math would suggest a year of birth as 1852. I did find John and Mary O'Neill, my second great grandparents in the 1851 Census of Canada West (as the current Province of Ontario was then called).
William was listed in that 1851 census as a one year old living with his parents John and Mary, along with an older sister, Hanora, aged 4, and John's mother and my third great grandmother, Hanora O'Neill (no maiden name known). John and his mother were listed as both being born in Ireland but Mary was listed as being born in Canada around 1828. It appears that Mary was about 18 or 19 years old when she married John who would have been probably about 33 or 34 years old.
Unfortunately, no additional information about the O'Neills can be found and the usual databases sources used don't seem to be able to help despite using various surnames variations and data 'mining tricks.'