If the 1916 marriage is correct the a birth for Margaret Clare in 1905 is extremely unlikely (an 11 year old would not be marrying). There seem to be 2 possiblities- either 1916 is not correct couple or ages of both Samuel and Margaret are much earlier than you thought.
The fact that the 1916 marriage names match and you know of children born 1917 and 1919 point to ages of Samuel and Margaret being earlier than 1900. It is, of course, possible that neither were living in Antrim or Down at time of 1911 census.
Think you need to get the 1916 marriage to see residences as well we what fathers' names and occupations were.