Hi, I was looking at the same birth record as you for Patrick Cassidy and believe that it is indeed his with transcription error. The reason why I tink ths is that Nicholas and Isabella twice before him and also with Patrick had Margaret McAfee as a sponsor to one of their children. The other two were Annabella & Catherine.
I also found a couple of rcords which may be helpful.
Deaths:
Nicholas Cassidy
died 17 Sept 1871
Maghera
76 yrs old
married
informant: Isabella Cassidy
Robert Nelson
died 1 July 1870
Tubermore
76 yrs old
married
informant: Ann Connery
* I'm not 100% on this one but he may have died someplace other than at home and it is before Patrick & Annie's marriage
Births:
Nicholas Cassidy
bap: 3 Sept 1788
Drogheda, co Louth
Roman Catholic
father: James Cassidy
mother: Mary Black
Robert Nelson
bap: 13 April 1795
Fahan Upper, co Donegal
Church of Ireland
address: Luddan
father: Thomas Nelson
mother: Mary
I looked around for the marriage of Robert & Margaret but did not find anything. There are several wills for Robert Nelson on Proni that you might want to check out. Also on PRONI I found the following;
1766 religious census:
Robert Nelson
Magherafelt, co Derry
Established church
1740 Protestant Householders;
Nicholas Cassidy
Kilcronaghan, Loughinsholin, co Derry
* these may be distant relations
I hope something might be helpful, Kim