Carnmoney is mostly in the RC parish of Greencastle. Unfortunately their baptism don’t start till 1854 so you won’t find any record there if Susanna was born there in 1840 and was RC. You say you assume she was RC. I looked at the 1901 census of Ireland. There are only 25 people named POE in it. Only 3 were RC and the rest were nearly all Church of Ireland. Carnmoney Church of Ireland records start in 1788, so they might be worth checking.
Statutory death registration only started in Ireland in 1864. Prior to that there were very few death records. Only the Church of Ireland routinely kept any. So can be difficult tracing deaths prior to 1864.
I had a look in the street directories (on the PRONI site) but cannot see an Andrew Magill printer listed 1819 – 1866. There’s some Poes though.
There are no Poes or Magills in Carnmoney cemetery. However there are some McGills (a common alternative spelling of Magill).