It's easier if you post a link to the Census.
http://www.census.nationalarchives.ie/pages/1911/King_s_Co_/Ferbane/Ferbane_Town/552558/
The census says married 29 years 11 Children 8 living
so still one to find.
BTW
The title of the thread reads Johnson not Johnston, you can change this by using the modify button.
I think all eleven are actually there
1 Thomas bap. Feb 20 1883
2 William (1901 census) bap Jul 18 1884
3 Patrick (1901 & 1911 census) b. c1885 bap.?
4 Bridget bap. Sep 28 1887
5 James (1901 & 1911 census) bap. May 12 1889
6 John (1901 & 1911 census) bap. Jan 27 1891
7 Catherine (1901 census) bap. Sep 20 1893 -
LINK Possibly Catherine in 19118 Joseph (1911 census) bc1894 bap.?
9 Mary Bridget (1901 & 1911 census) bap. Apr 19 1896
10 Peter (1901 & 1911 census) bap. Nov 07 1898
11 Margaret bc1901 - failte's records
Parents Peter JOHNSON/JOHNSTON and Catherine CORCORAN
married March 1882 Ferbane, Co. Offaly