This looks to be your John Charles Rennick in 1901:
http://www.census.nationalarchives.ie/pages/1901/Monaghan/Loughfea/Derry/1624557/
John Charles R married Margaret Sarah Cranston in 1897. His father is named as George, farmer, on the marriage certificate.
https://civilrecords.irishgenealogy.ie/churchrecords/images/marriage_returns/marriages_1897/10453/5806704.pdf
Family in 1901, with Gordon:
http://www.census.nationalarchives.ie/pages/1901/Monaghan/Loughfea/Derry/1624557/
I found Emily Sarah Rennick’s birth on 12.11.1872 at Lough Derg. Likewise Gordon Alexander on 22.4.1885; William Henry R on 16.2.1880; Ruth Julia R 24.4.1877; Sydney Charlotte R 2.12.1870; Harriet Eliza R 21.2.1869; John Charles 27.6.1867 & James Joseph 26.1.1866.
George Rennick & Sidney McDonnell married 1860, Rathdown, Co Wicklow.
Harriet married Charles Cranston on 18.7.1899 (so it looks as though that may be the same family that John Charles R married into). Harriet in 1901:
http://www.census.nationalarchives.ie/pages/1901/Cavan/Bailieboro/Rakeevan/1053498/
George R died August 1896 aged 47; Sidney died 2nd July 1897 aged 52; William Henry R died 21.10.1886 aged 1; Ruth died 11.5.1883 aged 5. You can view most of the certificates on this link (the 1860 marriage isn’t on-line free yet, so you would need to pay to see it):
https://churchrecords.irishgenealogy.ie/churchrecords/
George is listed in Derry townland (parish of Magheracloone) in Griffiths Valuation for 1861. He had plot 2a which was 212 acres, making it a very large farm for Ireland. So he was probably a gentleman farmer (as evidenced by having 4 servants in the 1901 census).
http://www.askaboutireland.ie/griffith-valuation/index.xml?action=nameSearch
George left a will. George’s estate was £1824, so a considerable sum at that time. The will is on-line on the PRONI wills site and mentions that the farm was 325 acres, so it had increased since Griffiths Valuation. There are various bequests including £300 to daughter Sidney Charlotte, so if there had been an estrangement, they had resolved it by that time. Other children get bequests too. Emily appears to have married and is Emily Ledney.
https://apps.proni.gov.uk/WillsCalendar_IE/willsSearchResultsDetails.aspx
Probate of the Will of George Rennick late of Lough Derry Carrickmacross County Monaghan Gentleman who died 31 August 1896 granted at Armagh to John Charles Rennick of Lough Derry Carrickmacross Gentleman.
Noted this:
Letters of Administration of the personal estate of George Thomas Rennick late of Lough Derry County Monaghan Gentleman who died 23 June 1893 at same place were granted at Armagh to George Rennick of Lough Derry Gentleman Farmer the Father.
Sydney died intestate, and her probate file was destroyed in the 1922 fire:
Administration of the estate of Sydney Rennick late of Lough Derry Carrickmacross County Monaghan Widow who died 2 July 1897 granted at Armagh to John C. Rennick of Lough Derry Gentleman Farmer a Son.
