Maureen,
The address where George lived was the townland of Ballyhill Upper. There were no house numbers or street names in 1851 and the townland alone was sufficient to identify you or get a letter delivered. I do not know the modern house number but the property is off Ballyhill Lane.
George’s father was John, a farmer. Griffiths Valuation for Ballyhill Upper in 1862 lists John Johnson on plot 10, a 13 acre farm. The map that accompanies Griffiths shows you where the farm was/is. It’s at the far end of a dead end lane off Ballyhill Lane. That in turn is off the A52 road from Nutts Corner Roundabout to Belfast.
Looking at the property today on Google Earth there is still a farmhouse and outbuildings on the site. There is a car parked outside which suggests it’s still occupied. Use the maps on the Griffiths site and then once you have identified Plot 10, use the slider bar in the top right hand side of the screen to overlay a modern map which will show you where the property is today.
http://www.askaboutireland.ie/griffith-valuation/index.xml?action=nameSearch
Probate abstracts:
The Will of William Johnson late of Ballyhill in the County of Antrim Farmer deceased who died 3 March 1864 at same place was proved at Belfast by the oaths of John Bell and William Hannah both of Carnachlis (Crumlin) in the County of Antrim Farmers the Executors.
Probate of the Will of John Jackson late of Ballyhill Ballycarry County Antrim Farmer who died 17 January 1901 granted at Belfast to Hugh Jackson Daniel Jackson and James M'Allister Farmers.
The above wills are on-line on the PRONI wills site. William Johnson’s will contains the helpful information that he wanted a gravestone erected at Loanends Presbyterian graveyard. So that is probably the family church.
Johnston John McComb of Ballyhill Crumlin county Antrim stockbrokers died 31 August 1926 at Royal Victoria Hospital Belfast Administration Belfast 2 November to James R. Johnston P.E. teacher. Effects £321 16s. 10d.
Johnston James Rea of The Bungalow Ballyhill county Antrim retired school teacher died 9 January 1936 Probate Belfast 26 February to James Johnston commercial traveller and William Robert Thompson joiner. Effects £430 11s. 3d.
John who died in 1926 was intestate so no will, but there should still be a probate file with details of next of kin. James does appear to have had a will which should be in PRONI in paper format.
1901 census for a George Johnston born c 1830:
http://www.census.nationalarchives.ie/pages/1901/Antrim/Dundesert/Ballyhill_Lower/917279/
And nearby:
http://www.census.nationalarchives.ie/pages/1901/Antrim/Dundesert/Ballyhill_Upper/917299/
Looking at the birth certificate for George Johnson on 7th Feb 1875 at Ballyhill, I see his mother was Rebecca Auld. You mentioned that your ancestor went to the USA c 1850. The George in the 1851 census appears to have still been in Ballyhill in 1901, and had been married to Rebecca Auld. Are you sure you have the right family?
I looked for a George Johnson marriage to Anne McGuiness in Ireland c 1850 but did not see one.