Location 55 23 38N, 4 06 07W.
Over Cairn and Nether Cairn are clearly two different properties as can be seen on the map linked above.
Scotlands Places of 1855/57 describes Over Cairn as a small farm house the property of the Marquis of Bute, occupied by John Howatson. The authority for spelling is George Muir of Nether Cairn.
Census
1841 Not found
1851 Not found
1861 Not found
(see March - the Howatsons are living there for above censuses. Was March perhaps another name for Over Cairn? See Probate below)
1871 John Howatson 72 farmer of 15 acres with wife Jane 70 and grandson David Paterson 3. Elizabeth Findlay 15 is their general servant.
1881 Over Cairn - John 81 and Jane K 79 are farming 15 acres with son in law William Paterson and Mary G Paterson 42, and children David 13, John H 12, Jane K 7, and Mary K G 4.
While also listed as Over Cairn are James Stevenson 48, farming 570 acres, 150 arable employing 1 man, 1 boy, and 2 women.( I believe this is a transcription error in the Find My Past database as James Stevenson lived at Nether Cairn.)
There are no more listings for Over Cairn in the census returns.
The Ayr Advertiser carries and announcement of the death of John Howatson aged 85 at Over Cairn with a request to copy it to the Australian newspapers. As can be seen from the Cumnock Connections website, several family members had emigrated.
Probate 21/12/1883 Ayr
Conf. of John Howatson, cottar, March New Cumnock who died 21/10/1883 at March, aforesaid, intestate, granted to Mary Howatson or Paterson wife of William Paterson, carter, March Aforesaid daughter of deceased. Executrix, next-of-kin.
1891 Nether Cairn James Stevenson 58
1901 Nether Cairn James Stevenson 68 with wife Annie 65,
Valuation Roll
1855 to 1875 tenant John Howatson, Lands of Over Cairn (no mention of a farm house)
1865 prop The Most Noble the Marquis of Bute John Patrick Crichton Stuart.
1885 tenant James Stevenson.
1915 tenant Allan Stevenson.
1920 prop James Campbell, tenant Allan Stevenson.
1925 tenant William Lennox.
1935 tenant William MacCrindle.
