Brian,
The spelling of a surname and why it changed is something that is discussed regularly on genealogy forums. As far as Ireland is concerned most people were barely literate and completely indifferent to how their name was spelled. Here are 2 examples of spelling varying within the same family in the same census:
http://www.census.nationalarchives.ie/pages/1901/Antrim/Sharvogues/Drumsough/920148/
http://www.census.nationalarchives.ie/pages/1851/Antrim/Upper_Glenarm/Carncastle/Four_Score_Acre/5/
In the 1901 Irish census there were 10478 people with the name Healy. 10259 were RC and 219 other denominations. 322 were spelled Healey, of whom 276 were RC and 46 other denominations. There were also 32 Hely, and probably there are other spellings if you search for them. It varied across the denominations, and across the country. According to an expert on the subject, the likely reasons for these variations are “local dialect, pronunciation and inconsistency in spelling, well into the nineteenth century.” (Maxwell - see below).
I have attached a couple of pages taken from “Your Irish Ancestors” by Ian Maxwell (2008) which covers the subject and refers to Matheson’s 1901 report on surname spelling variations. He highlights the fact that - as I mentioned above - it can vary within the same family, never mind around the country. That has to be because of whim or indifference rather than some wider strategic reason. If there's more of a particular spelling in a certain area, then that is probably down to local dialect.
It comes up in place names too. In Co Armagh there’s a town named Tandragee, and sometimes Tanderagee. Another named Belleek and Balleek. Both versions are in regular use. The residents don’t seem to lie awake at night wondering why there are 2 versions. Though possibly delivery drivers do.