If you look at Griffiths Valuation there are several Robert Trimbles so you would have to decide which may be yours. In Griffiths John Cowan appears in the townland of Corlis,parish of Creggan,county Armagh.Also in same place are John Cowan Jr,Michael,John and Patrick Cowan who are probably all same family.
In the Tithe Applotment books 1823-1838,I could find no listing for Robert Trimble but did find John Cowan who was tax assessed in 1827 in townland of Carlifs(Corlis),parish Creggan,county Armagh.
In 1901 census I didn't locate Robert Trimble(the father) but did find John Cowan age 75,roman catholic,born county Armagh,farmer,widower,living at Corlis,Cullyhanna,county Armagh.There is also a John Cowan age 48 and a John Cowan age 9 both at same address.
Lastly,this may be your Margaret.Margaret Trimble born @ 1813 and died Jan-March 1893 in Belfast.
On PRONI there is a will for John Cowan who died 5 March 1907 if you wish to look at it.