Improper spellings and recordings of surnames are frequent. The results can be confusing and misleading. Although this is a common problem in Genealogy, and Irish Genealogy, I'll give an example for my name (Reidy) which may be more or less confusing than some.In various family records I've seen the name spelt as Reidy, Riedy and Ready.
For example, READY is a common spelling variant of REIDY (derived from O'Riada) but it may also be used for REDDY (derived from O'Rodaigh). If you know something of the geographic origins of the family, that may help sort things out in a specific case as the O'Riada are a Dalcassian sept found mainly in the west of Ireland whereas the O'Rodaigh are based in Kilkenny. REEDY may be just a spelling variant but is often derived from Mac Riada (in Donegal) or Mac Conriada (in Derry). More details (and a fuller and more accurate explanation) can be found in MacLysaght's The Surnames of Ireland. Other surnames also seem to be mixed up with REIDY. To say the least, the situation is very confusing
To add an international dimension to the problem, there are similar or identical names with non-Irish origins. You will find several German/Swiss families named REIDY, RIEDY, or REEDY.