Thanks to Jumper O'Kelly over on the Donegal Friends Facebook Group page for this link to old photographs of the Old Abbey. These may have been taken around the 1890's or 1900's.
See here for the pics, there's a whole scatter of them.
I thought the photograph I below was interesting as that detail on the stonework in the foreground isn't there today.
And another, looking down Main Street toward the Diamond from where the entrance to the Car Park is today.
Original Posting on Donegal Friends.