Origin of a surname

The 1641 depositions (TCD website) mention a Phelim Cassidy from Fermanagh, so the surname was clearly in use then.  And I would expect it to have been in use for some considerable time before that. MacLysaght's "The Surnames of Ireland" says the Cassidys were traditional physicians to the Maguires in Fermanagh. So a long standing connection.

In my opinion Cassady is a variant of Cassidy. In the 1600s and earlier, people did not bother about accurate or consistent spelling and it changed all the time. That's self evident if you spend any time on baptism and similar records.

Wm,

People do sometimes suggest that a particular spelling of a name indicates a particular religious denomination. I have not really found that to be the case. I had a look at the 1901 census of Ireland. There were 5528 Cassidy entries. 5026 were RC, and 502 were Church of Ireland, Presbyterian, Baptist and other protestant denominations. There were just 3 Cassady listed. All were RC. So, in 1901 anyway, there were no Protestant Cassady households at all, and all the Protestants spelled it Cassidy. 

The idea of a single or correct spelling for a surname or a place name in Ireland is very much a recent phenomenon designed to meet the needs of modern officialdom. Before that there was no consistency. Names were spelled phonetically and each variation was down to the whim of the particular person recording the information. You will often see the spelling change as the records go back. This rarely indicates a deliberate decision to alter the name, nor even a mistake. Not everyone was literate, but even when they were, exact and consistent spelling simply wasn’t something they bothered about. In addition to varying the actual spelling, O’ or Mac prefixes were optional and were often omitted. 

In 1899, the Rev Smith reviewed the early records of Antrim 1st Presbyterian church (covering the years 1674 to c 1736). He noted: “Even the same word is not always spelled alike by the same hand. Indeed spelling with most of the recording officials (and they must have been fairly numerous) was a matter of the most sublime indifference. The name William, for instance, is spelled 3 different ways in as many lines; while Donegore, a neighbouring parish, is spelled 10 different ways; but these extend over a good number of years. Many families names are spelled phonetically, while others are given in the most round-about fashion”.

Here’s an example from the 1851 Irish census where the spelling of the surname varied within the same family (census return completed by the same person).

http://www.census.nationalarchives.ie/pages/1851/Antrim/Upper_Glenarm/Carncastle/Four_Score_Acre/5/

I can easily find others like that.

Another element to consider is that Cassady (and variants) is an anglicised version of a gaelic name. (MacLysaght gives it as Ó Caiside). In Irish (gaelic) the spelling and prefixes vary depending firstly on what case is used (eg genitive usually requires the insertion of an extra “i”), and secondly with a woman’s name, it changed according to her marital status. It is a further factor in explaining why no-one in Ireland worried about the “correct” spelling. There wasn’t one. This bilingual nature of many Irish names is an important factor in the variations in spellings.

Can’t really help you with the first occasion that Cassady may have been used.  PRONI has a document D607/A/127 dated 24th Feb 1755 which is a letter from a Robert Isaac and he mentions some tenants by name. One is a John Cassady.  Perhaps someone else reading this can find you an earlier example?