Leathem,
I can confirm there is a death registered in Armagh on 16.1.1873 for John Cassidy, aged 71. That record is not viewable on-line free at present but you can pay to view it on the GRONI website, using the “search registrations” option:
https://geni.nidirect.gov.uk
You will need to open an account and buy some credits. It costs £2.50 (sterling) to a view a certificate.
The certificate should tell you where John lived, and also whether Susanna was still alive. If you post details of where he lived (his townland) and his denomination, I’ll try and suggest where he may be buried. Knowing his occupation may help too. If he was a labourer, for example, he probably won’t have a gravestone, but a farmer is more likely to.
Death registration started in 1864. I searched for a Susanna Cassidy death 1864 – 1901 in Armagh but did not see one. Possibly she died prior to 1864, in which case there may be no record of her death. Her husband’s death certificate should tell you whether she was alive then.