The ownership of land is recorded by the Land Registry (in Dublin for Co Antrim deeds prior to 1922. You have to pay a fee to search). There is no requirement to register leases, as far as I know. Each party to a lease keeps a copy but what happens to those documents after the lease expires is anyone's guess. Some people might keep the lease with the deeds but I don't think there is a legal requirement to keep it. You could try writing to the current owners of the land to see if they know, but I wouldn't be too optimistic after all this time.
By the way, are you aware that Killead is in the small part of the 1851 census which has survived. If not, you can look at it on:www.homepage.ntlworld.com/derek.hawthorne/Antrim5113.html