The act wasn't in place at the time the census was taken.
https://www.irishtimes.com/opinion/an-irishman-s-diary-1.1081733
You can argue that the rule still applies as it was put in place later but not that the people in 1926 were given that promise.
You have been misinformed.
The 1926 census was performed under regulations made in accordance with the Statistics Act, 1926.
The Statistics Act is available online. Section 13(1) gives a time unlimited commitment that the information gathered under the Act would not be published without the consent of the individuals concerned.
The text of the relevant Section is as follows:
"13.—(1) No individual schedule, form, or other document filled in or otherwise completed by any person in pursuance of any requisition made under this Act and no part of any such document and no verbal information or answer given relating to any individual person, business or concern shall without the consent of such person be published or, except for the purposes of a prosecution under this Act, be shown or communicated to any person other than an officer of statistics concerned therewith in the course of his duty as such officer."