Civil registration of deaths started in 1864- if she died after that date you can search the index for the details needed to order the death certificate- remember that Agnes/Nancy/Ann are all used as forms of the same name. The Irish BMD Index is available at this
link.
If she died before 1864 then you need to check local records (gravestone inscription, church records (although most churches didn't keep burial records), newspaper notice, etc.
