I'd start by searching for the marriage of Peter & Martha - presumably in England (check to see where Martha and the various children were born for clues to where the marriage may have been). The 1911 census should say how many years they are married.
A search for a marriage is usually easier if you have the bride's surname to cross-check any possible matches, so it might be a good idea to start with a birth cert for one of their children to establish Martha's maiden name, if you dont already know that.
A marriage cert would give you their father's names and occupations to help narrow down possible birth matches.
(ages on Census returns etc are not always accurate, the the extra detail can help confirm, or rule out a possible match)
The BMD Index for England/Wales is on FreeBMD
The Irish BMD Index is available on FamilySearch
(both are also available on Ancestry)
You can use the references on the indexes to order certs from the relevant GRO.
Have you located Peter on any earlier census returns in England ?
He may be with other family member or even his parents in 1901....