You say that John Cassidy died on 16th Jan 1874 in Armagh. I can’t find a death that matches that information. There was one exactly a year earlier on 16th Jan 1873, aged 71, however he was a bachelor.
How sure are you about your information?
Probably also worth mentioning that there are several graveyards in Armagh that a Catholic might be buried in. There’s one at St Patrick’s cathedral, another at Sandy Hill and a third at St Mark’s. But some families have plots in graveyards further out of town (perhaps where they used to live) and so tracing a grave can be hard going without accurate information as to which graveyard they used.