I am sorry to see that you have not gotten a reply at this stage on your query for Long family in the Killygordan Area. It is quite possible the that the timeframe that you happen to be interested in is just too far back for Records to be available to help research and tie family connection together. This link will show you the availability of records throughout County Donegal with regards to various locations and their Religious interests.
COUNTY DONEGAL Roots Ireland List: Years that available Registers & Records started to be recorded in Co Donegal
I am afraid I have not come across any books either on the History of the Killygordan Estates, all I could find online was a small bit of information into the Building itself. https://www.buildingsofireland.ie - Killygordon House
There are a few sites that have interesting information posted for Killygorden in the Parish of Donaghmore which are worth looking up.
I am sorry we couldn’t be of more help to you, it is just the timeframe that is catching me at this time.
Kind Regards Seamus Callaghan
Volunteer for Parish of Clondavaddog