Six Mile Cross, Inishative would be in the parish of Termonmaguirk (Both a Civil and a Catholic Parish) see http://www.irishtimes.com/ancestor/fuses/townlands/index.cfm?fuseaction=... and
Catholic Church records for Termonmaguirk date back to 1834 and are available from the National Library of Ireland in Dublin , Public Records Office Northern Ireland and for a fee from http://www.irish-world.com. They should also be available from the local church.
Also have ave a look at Griffiths Valuation at http://www.askaboutireland.ie/griffith-valuation/
Kind regards,Rory O'Shaughnessy | Community Activation Leader Ireland Reaching Out