Tricky doing anything this recent. I think the confusion may start with Laurence O'Brien marrying Mary Sheehan on 16th November 1940.
That would mean that the following are probably yours:
Frank O'Brien Jan - Mar 1934
Catherine O'Brien Apr - Jun 1935 Probably died 1935
Bridget O'Brien Apr - Jun 1936 Probably died 1936
Ellen O'Brien Oct - Dec 1937
John O'Brien Jul - Sep 1938
Bridget O'Brien Apr - Jun 1940
Whereas the following could belong to either family:
Catherine O'Brien Apr - Jun 1941 Probably yours, unless Mary Sheehan was pregnant when she got married
Peter O'Brien Oct - Dec 1942 } One of these will be yours
Peter O'Brien Apr - Jun 1942 }
Patrick O'Brien Jan - Mar 1944 } One of these is likely to be yours
Margaret O'Brien Oct - Dec 1944 }
Kevin O'Brien Apr - Jun 1945 Unlikely to be Margaret's brother
Catherine O'Brien Jan - Mar 1946 Second Catherine, not likely to be yours
Michael O'Brien Jul - Sep 1946 Too close to Catherine above, probably yours
Bridget O'Brien Oct - Dec 1947 Second Bridget, not likely to be yours
Mary O'Brien Jan - Mar 1948 Too close to Bridget above, probably yours
Anne O'Brien Jul - Sep 1949 Too close to Mary below, probably yours
Mary O'Brien Jan - Mar 1950 Second Mary, not likely to be yours
William O'Brien Jul - Sep 1950 Too close to Mary above, probably yours
Helena M O'Brien Jul - Sep 1951 Unlikely to have Helena and Ellen in same family
Alice O'Brien Oct - Dec 1952
Paul O'Brien Jul - Sep 1953
Christopher O'Brien 1955
Joseph O'Brien Jan - Mar 1958
Of course if there were a third O'Brien-Sheehan couple it could all get very complicated!!!