Many thanks to all on the forum for the help and input.
I can now post the following as the history of all the New Zealand Deyells, all descended from George and Margaret Deyell who arrived in NZ aboard the Mermaid in 1862.
His father was another George Deyell living in Kilsherdney, County Cavan. His mother was Sarah.
Fortunately for us it seems that Susan Deyell (a daughter of George Snr and Sarah born c1849) asked for an extract from the 1851-Census in 1915, in order to support her application for a pension. That extract shows George Snr and Sarah (and Susan) living in the townland of Cornanurney, parish of Kildrumsherdan, barony of Tullygarvey in 1851 - where the 1857 Griffiths indicates his landlord to have been a Robert Burrowes. So this would appear to be the home that George Jnr (who arrived in NZ) would have known in Ireland. It would seem that George Jnr also had a number of siblings, as George Snr shows up as father-of-the-bride for at least 2 daughters;
• Elizabeth - b 1832 - weds John Byers in 1854 before they head off to Australia.
• Susan - see above, born c1849. Marries John Conolly in 1872 (recorded as Susannah Dyell at wedding). Susan and John Conolly are in the 1911 Irish Census in Raw, Cormeen, Co. Monaghan with a 25yr old son, William.
• another possible sister, Alicia (b.1853), who heads off to Queensland in 1873 where she marries James Bullock, has 3 children and dies in Brisbaine in 1935 (Note that in her Australian wedding notice, Elicia is referred to as the "5th daughter" of George Deyell).
• See also the brothers Alexander (b.c1841) and Humphrey (b.c1851) referred to below.
• There is also a Sarah Deyell (20yrs) on the passenger list of the Mermaid with George and Margaret in 1862 - whos relationship to George Snr and George Jr is unclear, and who then seems to vanish in NZ.
It seems George Snr and Sarah remained in this area all the rest of their lives, until George died in November 1889 (aged c.85) followed by Sarah in early 1893 (aged c.84) - Sarah shows as Dayell in the death records. However the remaining family appears to have stayed on, as the 1901 Census shows as residents of Cornanurney (Tullyvin East, Cavan) one Alexander Deyell (60) and his brother Humphrey Deyell (50), along with a Sarah Connoly (66) - described as a servant (note there is another Alexander Deyell, born 1861 and married to Rebecca with at least 5 children, in nearby Bailieborough - who may be the son of our Alex). Only Humphrey survives in the family home by the 1911 Census - although in that Census he is said to 69 years old.
The strange thing is that there are continuous records of the family in Kilsherdney from the 1851 census, via a number of court cases in Cootehill involving George Snr from 1853 to 1863, the marriages of Elizabeth and Susan and George Jr, the deaths of George Snr and Sarah, and the 1901 and 1911 census for Alexander and Humphrey - but there are no records I can find for anyone before 1851 - and by no records, I mean no records that fit in Ireland, Scotland or England. The closest I can find seems to be a George Deyell who was a merchant seaman born on Shetland in the early years of the 19th Century and traceable only from his seaman's records of 1835/6.
So the mystery of earlier generations continues....
