I just went through a very frustrating experience with the GRO office in Roscommon, which was in large part my fault. I thought I would mention what I did wrong here so others can avoid the same error.
My gg-grandmother's name was Mary McCarthy, and she died in the Skibbereen district. Searching for anybody named "Mary McCarthy" is a major challenge, to say the least, and one I try to avoid as often as possible. I wanted some confirmation of her age, so I'd have some idea as to where to start looking for her baptismal record.
All I knew was that she died some time after the 1911 census. I (correctly) assumed she died in the home in Tooreen that had been her and her husband's (Cornelius McCarthy), then her son-in-law's (Humphrey Michael Collins), then oldest grandson's (Michael Humphrey Collins). Since my Aunt Peg does not recall her death, I also (correctly) assumed Mary McCarthy died some time before Peg could remember (before 1929). This narrowed down where to search civil registration to 1911-1929. For good measure I went up to 1933, in case Aunt Peg for some reason didn't know about her g-grandmother's death. I had a reason for doing this, as apparently there was some kind of rift between Grandmom and her brother-in-law Michael Humphrey Collins's wife, and it could have affected how the two families might have interacted.
This left over two dozen records to consider. And I ended up ordering every one of them.
There were 26 possible records. I then expanded my thinking and considered the scenario that she had gone off to live elsewhere with one of her daughters. Even with that scenario, I was still able to easily eliminate 22 records. Among the reasons - a son was present at death (Mary and Cornelius McCarthy had seven daughters, no surviving sons); she died in the Workhouse; the deceased was a spinster; she wasn't a widow (my gg-grandmother was); her occupation or her spouse's occupation was wrong (gg-granddad Cornelius was a farmer). In two records there was a daughter present at death, but I know the names of daughters who emigrated, so I felt those records weren't right either. That left one record - the one I kept asking for and kept not getting.
GRO skipped over my gg-grandmother's death record. The reason this happened was because there were two elderly ladies named Mary McCarthy who died in Skibbereen, four years apart in age, and in the same quarter and year. This means they were in the same volume, and not only that but they also happened to be on the same page. If you think about it, the two records will look almost identical. Since they were on the same page, the two deaths were registered in the same local office, and probably written up by the same person, so the handwriting is the same.
And this is where I went wrong. I should have noticed the multiple entries on the same page and called that out in my letters to GRO. I had sent one letter specifying that she died in Tooreen, but that letter actually got lost in the mail. Every time I asked for the other record on the page GRO kept sending back the erroneous one.
The Mormon Family History Library sent me an image of the index. In an image editor I boxed in blue the index entry of the record I already had with the words ALREADY HAVE. And then I boxed in red the index entry of the record I was missing, with the words NEED THIS ONE. I enclosed that picture along with an explanation in my letter to GRO.
That did the trick. I now have my gg-grandmother's death record. But my joy at finding it was mixed with great annoyance, when I noticed that the record number (#379) was incrementally the one immediately following the one GRO kept sending back (#378). Even in the Ireland Registry Office they can get confused. One record can look exactly like another.
If you are searching for a very common name like Mary McCarthy, be aware of these pitfalls. Make sure you keep all your record requests in a spreadsheet. This will help you notice if you have multiple requests on the exact same page of the registry book. And fill in all the information in your spreadsheet when you get back records.