There is only one William Keig whose parents are Thomas Keig and Ann Kelly--he was ch 06 DEC 1840 Patrick

William John ch 26 SEP 1841 Patrick father Paul Keig mother Elinor Cowle

There is really only one set of Thomas Keig/Ann Kelly-md 1834

The one reported in 1840 was by a Mormon church member--although occasionally these have proved to be accurate, I have generally found them to be quite unreliable.
I think one is on much firmer ground with parish extractions of parish records.

1841 Census Index
3 22 104
Kegg
Anne 27
Margaret 6
Thomas 30
Thomas 30 months
William 6 months

Thomas Keig died in 1843 and was buried in Patrick 17 Mar 1843
Ann Kelly Keig remarried John Crellin 20 Feb 1845 Patrick
1851 Census
Patrick
Glen Maye
John Crellin head married 58 corn miller b Patrick
Ann wife married 39 b Patrick
Henry son 13 corn miller b Patrick
Ann daughter 11 b Patrick
James son 5 b Patrick
Josiah son 3 b Patrick
Cath dau 1 b Patrick
Thomas Kegg wifes son 12 b Patrick
William Kegg wifes son 8 b Patrick
Ann Kegg wifes dau 7 b Patrick

Per IGI Thomas Keig and Ann Kelly had:
MARGARET KEIG Christening: 05 JUL 1835 Patrick
JOHN KEIG Christening: 19 MAR 1837 Patrick
THOMAS KEIG Christening: 25 NOV 1838 Patrick
WILLIAM KEIG Christening: 06 DEC 1840 Patrick
ANNE KEGG Christening: 17 MAR 1843 Patrick

John Crellin and Ann Kelly Keig had several children who would be 1/2 siblings to William