dbe,
Looking at Griffiths Valuation of Crossgar in 1863 there were 2 John Hanna properties. Plot 56 (27 acres) and plot 68 (5 acres plus some more land in plot 69). Usually when there are 2 people of the same name in a townland, the Griffiths clerks would use an agnomen (nickname) to distinguish different folk. If there is no agnomen then usually the one person holds more than one property in the townland. So I think John Hanna probably had two farms there at that time.
Searching the Valuation revision records, which take Griffiths forward, I see that John Hanna on plot 56 is deleted in 1887 and replaced by Samuel Hanna. In turn in 1910, Samuel is replaced by Robert Hanna. John Hanna on plot 68 is replaced by Joseph in 1871. Not sure of the connection there.
This looks to be Samuel in 1901:
http://www.census.nationalarchives.ie/pages/1901/Down/Crossgar/Crossgar__part_/1205174/
By 1911 the farm is held by Robert who had married Jeannie (Jane) Earls on 17.2.1908 in Saintfield 1st. The marriage cert confirms his father was John Hanna. So it looks as though the farm went from John Hanna senior to son Samuel, and when he died to another son Robert.
Robert and family in 1911:
http://www.census.nationalarchives.ie/pages/1911/Down/Crossgar/Crossgar…
John Hanna senior died at Crossgar on 29.2.1884 aged 62. Informant was son John. His wife Jane died 12.12.1892 aged 73. Informant was son Samuel. Samuel died 16.8.1907 aged 50. Still a bachelor.
So I can’t help you with what happened to the 3 daughters Elizabeth, Mary & Martha but I suspect that there were 3 sons around until the early 1900s and 2 for some time after that.
Deaths and marriages on this website:
https://churchrecords.irishgenealogy.ie/churchrecords/
Griffiths:
http://www.askaboutireland.ie/griffith-valuation/index.xml
Valuation revision records:
https://www.nidirect.gov.uk/proni
John Hanna junior in 1901, who has presumably acquired the second farm:
http://www.census.nationalarchives.ie/pages/1901/Down/Crossgar/Crossgar__part_/1205184/
John junior married Mary Jamison on 20.8.1879. The marriage cert confirms his father was named John.
Some probate abstracts from the PRONI wills site:
Probate of the Will of Samuel Hanna late of Crossgar Dromara County Down Farmer who died 16 August 1907 granted at Belfast to William Poole Farmer.
The will itself is on-line and Samuel left everything to Robert.
Hanna Robert of Crossgar county Down farmer died 21 October 1926 Administration Belfast 16 February to Jane Hanna the widow. Effects £573 18s. 9d.
John & Jane’s children were all apparently born before 1864 so you would need to check baptism records to make sure these are all the correct family.