It cannot be stressed enough that the researcher should not be too hung-up about the spelling of surnames. Within the same family you can find children baptised as Smith and registered as Smyth, to take but one simple example. Names that in the 20th Century were always spelled with a prefix of O' were often spelled in the 19th Century without it, e.g. O'Neill and Neill.
The surname Kelleher is one I have seen rendered in about 7 different ways - to name but some:
Another one designed to catch the unwary is the surname Kane. Depending on the recorder, it can be rendered as Kane, Keane, Cain and Caine.
As for the surname Colreavy (sometimes Culreavy) it was synonymous with Gray, and within the same family either variant could be used - thus some children were registered as Gray with siblings registered as Colreavy.
Any researcher is advised to study Edward MacLysaghts The Surnames of Ireland for more information. See also chapter on Family Names in my book Tracing Your Leitrim Ancestors.
Again, in general, although I've noted the variants in many cases, in others I have not. I have also tended to keep the first variant of a name found in a family and used it as the main surname for all members of that family, including all descendants.