Hi - checked on (pay) site for children of WILLIAM COATES and MARGARET, and these details came up:
Action Cost Source Surname First Name Year County
25 credits Church Baptism Coates Samuel 1844 Co. Dublin
25 credits Church Baptism Coates Maria 1845 Co. Dublin
25 credits Church Baptism Coates William 1846 Co. Dublin
25 credits Church Baptism Coates Martin 1849 Co. Dublin
25 credits Church Baptism Coates William 1851 Co. Dublin
Perhaps son Thomas who married in 1868 just wasn't registered. You'd have to purchase at least one of the above certs to get Margaret's surname, and from that, perhaps a marriage to William Coates + fathers of both. Of course, there's no guarantee that it's the right William & Margaret!
I wonder if this marriage of Maria Coates with father William is a daughter of William & Margaret?
Ireland, Select Marriages, 1619-1898
Name: Maria Coates
Marital Status: Single
Marriage Date: 12 Feb 1866
Marriage Place: Kilcook, Kildare, Ireland
Spouse: James Hendrick
FHL Film Number: 101488
Reference ID: P642cn29
It doesn't appear that the Coates of KNOCKANALLY are related to your Coates, as this link gives the family history, which is closely related to the BOMFORD and WEBB families http://www.bomford.net/IrishBomfords/Chapters/Chapter8/Chapt...
Have you seen COATES Wills at this site? William Lancake Coates of Knockanelly left Knockanelly to his nephew, William Coristine Coates http://www.willcalendars.nationalarchives.ie/search/cwa/inde...
Think that's enough for now! Good luck and happy hunting