My own surname is a good example to take of how a Gaelic surname has been corrupted over the centuries of English administration. Quinn is an anglicised corruption of the Gaelic surname O'Cuinn (see above), and the variations of the surname that have emerged over the years illustrate the anglicising process.
Over a period of five hundred years one comes across Ua Cuinn, O'Coyne, O'Quyane, O'Quin, Quin and finally Quinn as well as other unmentionable corruptions.
In the Calendar of State Papers for the period the sept is referred to as O'Quins, Quins, O'Quynnes. One man is referred to as Neale O'Quyn, O'Quin, O'Quinn, O'Quyne, O'Quynn, O'Quynne, Quyn (rather than as Niall O'Cuinn).
In the State Papers for the mid seventeenth century the prefix O' is dropped and the name is represented by Quin, Quiny, Quine, Quynne.
From the end of the seventeenth century until the end of the eighteenth century, O'Quin is the most common form in the documents although other corruptions can still be found, though to a lesser extent than the immediately preceding period, examples of these are: Quin, Quayne, O'Quyne.
With the nineteenth century the prefix O' came finally to be dropped in English, Quin was now the most common form of the surname but before the middle of the century it had been surpassed by Quinn.
From an examination of Griffith's Land Valuation circa 1850's it is obvious that Quinn had surpassed Quin by at least two to one. The practise of adding the extra n continued right up to the end of the century.
As has already been noted the vast majority of the Irish people in the past were unable to read or write, but when a situation arose, where it was necessary for the surname to be written down, it was written down in a contemporary form.
On the other hand literate people were certainly not going to be told how to spell their own surname, and would write it as they would have understood how their forebears had done with only one n.
My grandfather born in 1891 was registered as John Quinn·, his father born in 1866 was registered as James Quin.
In my own case though registered as John Quinn (being called after my grandfather), I am known as Sean Quinn,though in many official documents I am Sean O'Cuinn.