The Morris Tribunal will today examine whether politicians should be able to keep their sources of information private.
The inquiry insists Labour TD Brendan Howlin and Fine Gael Senator Jim Higgins should hand over records of information they received about allegations of garda corruption in Donegal.
The politicians argue that such information is privileged under the constitution.
They are refusing to hand over records they have received about allegations of garda corruption.
The tribunal has been told by lawyers that the two houses of the Oireachtas, the Dail and Seanad, have the power to protect the papers of TD's and Senators.
Speaking last week on behalf of himself and Senator Higgins, Brendan Howlin said they are both willing to give the tribunal all the information they have, but neither is prepared to divulge their sources.
Mr Howlin argues that as long as they have handled the information properly, they have privilege over it.