Hello Bill, I am looking at old queries and yours is tricky, at that remove we use church records so religion assists pinpoint where to look. I searched a subscription site and found no baptism for McGarvey in Tyrone, any McGarvey. Catholic baptisms for Omagh only commence in 1848
https://registers.nli.ie/parishes/0365
There was a tax on landholders of over an acre called the Tithes and applied to tenants also, there is no sign of any McGarvey in Tyrone (always possible they have been mis transcribed and put under the incorrect county) there are McGarvey names in surrounding counties mainly Donegal. It was an unpopular tax to support the Church of Ireland, the Establishment Church, they did have various responsibilities for their areas as in civic. It was done from 1824 to 1838 I think. See http://titheapplotmentbooks.nationalarchives.ie/search/tab/home.jsp
Now another land survey Griffiths Valuation was done and in 1858/59 in Tyrone and there 66 McGarvey entries with boSee th landlord and tenant recorded and a map. https://www.askaboutireland.ie/griffith-valuation/index.xml?action=name…;
I did not locate a marriage either and searched country wide. Same for a baptism, there are 8 John McGarvey in Tyrone but none 1840 to 1850 and the others have no the correct parents and only 3 could possibly be Mary's but latest baptised in 1838.
Looks like unless new records are located you are not going to find the information.
Regards
Pat