Department of Defence

I am researching the long and distinguished career of LtGen Peadar MacMahon, first as Chief of Staff between 1924 - 1927 and then as Secretary of the Department of Defence between 1927 - 1958. 1I have already consulted files in Military Archives for his period as Chief of Staff and now wish to research his 'second' career. I would like to inquire if there exists a series of files linked to the Secretary, distinct from the general registry files. If there is such a series I would be very grateful for permission to consult it.