When Fianna Fáil was founded in 1926 by veterans of the losing anti-Treaty side in the Civil War, abolishing the oath was a core aim.
It was a main item in the manifesto for its successful 1932 general election campaign, after which it formed a minority government whose first action was to introduce the Constitution (Removal of Oath) Bill 1932.
Seanad Éireann had more ex-unionists and others conciliatory towards the United Kingdom, and voted to reject the bill unless the Treaty could be amended by agreement.
After the 1933 general election, the Fianna Fáil majority government was able to override the Seanad and enact the law.
In 1935 the Judicial Committee of the Privy Council in London ruled that, in British law, the Oireachtas did have the power, under the Statute of Westminster 1931.