Source: son's marriage certificate, where he gives his address as Letterloan (Pa of Macosquin). Also given in daughter's marriage entry (1869) in Coleraine Local Record Office where her address is also given as Letterlone.
There are no McKeowns on the 1740 Macosquin Protestant Householders list.
A search of the 1831 Census films at Derry FHC found that Letterloan (and a dozen other townlands in Macosquin) was missing. Bill McAfee's database does not have any McKeons in macosquin either.
There are no McKeowns in Letterloan in the Griffiths Valuation BUT, there is a John McKeon in Lower Kiltinney, where this John's son, also called John, is living when he christens his own son, yet again a John, in 1876. This house is Plot 9Ba and is a ten shilling house with a large garden. I do not seem to have the GVRBs for Lower Kiltinney to see what happens to this man.
However, a house called 1d, and then 1Ad, has the following tenants in the Griffiths Valuation Revision Books for Letterloan: Thomas Dooey becomes Widow Dooey in 1881, James Campbell in 1886 and John McKeown in 1889. It then becomes John McKeown Jnr in 1892, by which time there are two more houses under 1d, the third one belonging to another John McKeown which becomes Mrs John McKeown in 1892. In the meantime, a John McKeown replaced Henry Hart in House 2c in 1885 and then is himself replaced by a John Hall in 1893. House 1Ad moves from John McKeown Jnr to Martha McKeown in 1906, while the third house, now designated House 1Ac, moves from Mrs John McKeown to Mrs Mary McKeown in the same year. It is in ruins by 1909 but Martha continues to live in 1Ad up until the GVRBs cease in 1927. Does all this suggest that this John McKeown was the one in House 2c because we know he died in 1891.
As well as the John, Margaret and James already given as children of this John, there are records for a Jane McKeown marrying in 1881 and a Rebecca Anne marrying in 1883, both from Letterlone and both marrying in 2nd Dunboe - BUT - their father John McKeown is given as a farmer rather than a weaver, and 1881 is a little late for children of John, BUT, a little bit early for the children of this John's son, John!!