Born "Ireland" or "Britain" (from his son's immigration records to US in 1912)
Nationality "British" (from grandfather's marriage certificate 1912)
Son: Ernest Davis (b. Beechworth, VIC 1872)
Ernest's father is listed as "John Robert Davis" from "Britain" or "Ireland" on all documentation we have.
The whole of Ireland was part of the United Kingdom when John Davis and Ernest Davis were born so John Robert Davis was a British citizen.
Civil registration of births began too late in Ireland (1864) for the birth of John Robert Davis to have been registered. Civil registration of marriages, except those in Catholic churches, began 1845.
If John Robert Davis was born in Ireland prior to 1864 the only record made around the time of his birth would have been an entry in a baptism register. You need to know where in Ireland he was born & approximately when + his religious denomination before searching for his baptism. Existence of baptism registers varies.
Some Irish people moved to Britain to live & work before emigrating long-distance.
Davis is a common surname in Wales. There was a significant ethnically Welsh population in Ireland for centuries.
On the other hand, parents of John Robert Davis may have lived in Ireland temporarily. There was much traversing the Irish Sea, particularly between Wales & S.E. Ireland but also other parts of Britain from all Irish provinces. Miners, farm labourers, construction workers moved for work.
There was a large British Army presence in Ireland in 19th century, including regiments from England, Scotland and Wales. Army chaplains' regimental births registers predate the start of civil registration of births but again, coverage is variable.