That is disappointing but hopefully won't happen again now you know! Always buy/copy the complete certificate & do not snip too close to the lines as sometimes people/priest wrote snippets outside the columns. If the certificate was subsequently altered the regulation/reasons are usually noted outside the certificate
The reason it is important to find out who registered the death is that if it was registered by an official say from the Workhouse/hospital they may not have really had any idea and may have been going on what Margaret or a younger relation told them. If her death was registered by a relative then if it was say a son they might have a better idea than say a grandson. Ages were not seen as important in thse days as we view them now.
The idea about the last illness is to see if she was suffering from a short or long term illness. She may have been admitted in extremis and died a few days/weeks later. In that case she may not have been able to give a coherent date and they may have relied on someone saying 'oh she was pretty old...probably in her 90s'. Or she may have had a long standing debility and gradually faded away.
The idea of interrogating a death certifcate in this way is to see if the age is correct on the balance of probabilities/how much reliance you can place on it.
Are you not able to view as opposed to having to purchase?
Could you pl advise the place name where she died.
ETA is it this place? do you think?
https://www.townlands.ie/londonderry/loughinsholin/tamlaght-ocrilly/clady/moneystaghan-macpeake/We have had this census up before of a widowed Margaret living in House 44. In 1911 she gives her age as 68. ETA I have clarified this sentence
Add the 21 years from this record to the date of the death notice and you get 89 years. http://www.census.nationalarchives.ie/pages/1911/Londonderry/Clady/Ballymacpeake_Lower/608661/In this record 1911 there are 2 other Margaret Marrons in the same location.
The niece (Maggie) 35 (though it does look a bit like 85) of Hugh Marron and 9 month old twins called Mary & Margaret Devlin and noted as relatives.
http://www.census.nationalarchives.ie/reels/nai002860484/Thye could be named for the older Margaret the seamstress aged 68.