Lemass and his immediate family
Seán, known as John or Jack to his family, was the second son of John T. Lemass and Frances Phelan. He was born in July 1899 in Ballybrack, Co. Dublin, at a summer cottage the Lemass family rented regularly. Click here for his birth index entry. If you click through the entry you'll get to an image of the copy-register with full details of address and occupation. His elder brother Noel was born in 1897 and baptised in the Pro-Cathedral.
Notice that Noel's surname is mistranscribed. This is a very frequent event with uncommon surnames. On the one hand, they are conspicuous and easy to find; on the other, record-keepers (and transcribers) are unfamiliar with them and so make mistakes.
This is what happened with the family's 1901 census return. In 1911, they are easy to find, listed under 'Lemass' - but the 1901 transcript has them as 'Lemars'. If you look at the original return, the handwriting of John, Seán's father, explains why.
Between the two returns, the family list five children, Noel (1897), John ('Seán'), (1899), Alice (recorded as 'Mary' in 1901), Clara (1904), Patrick (1906), Mary ('Mai' 1910). Two children appear to have died in infancy, Nora (1903) and Winifred (1905). The state birth indexes record mother's maiden name after 1900, revealing that two other children were born after the 1911 census was taken, Frances (1911) and Herbert (1914), the baby who was shot in January 1916. In all, then, Seán's parents had (at least) nine children.