Anna, I didn't find a marriage listed for these two in any of my books. I too found them on the 1880 census and on the 1900: 1900 North Overton, Owen, ed 49, p. 15b William Hearn, 8/1849, married 23 yrs., farmer, Ky Miranda, 5/1850, she has had 5 children, 5 are living george, 6/1878 Birdie, 4/1881 Willie, 2/1886 Robert, 1/1888 Mary, 5/1890
It looks like it says Miranda and not Lorinda. It could be a census taker error. It say they were married about 1877 which fits with Allice, the first daughter. What I would suggest is that you write to the Owen Co. courthouse and see if they have a marriage record. Owen Co. Courthouse PO Box 338 Owenton, KY 40359 Be sure to include a SASE so they can respond easily.
You might also get copies of their death cert. It won't give you any marriage info but it might give you other interesting information. Lorindia's is #18337 Vol. 37 8/9/1921 and William's is either #15586 Vol. 32 2/21/1931 or #15378 Vol. 31 5/4/1917. Neither of these Williams seem to be the right age. If you go to the website below a very nice woman will make copies of the death cert. for about $3 each. http://kyssar.hypermart.net/kyvital.htmhttp://kyssar.hypermart.net/kyvital.htm