If you enabled IRM on the Document Library, you can simply disable IRM on the Document Library. Documents that reside in a Document Library with this method of enabling IRM are not encrypted with AD RMS while the document itself resides in the library -- they are encrypted when the document is opened.
If the document itself was IRM'ed via Word, on the AD RMS cluster, you can add one or more users as super users who 'override' the right applied to the document.