According to the passage, the IRS's chief legal advisor is

The IRS Chief Counsel is appointed by the President of the United States, with the advice and consent of the U.S. Senate, and serves as the chief legal advisor to the IRS Commissioner on all matters pertaining to the interpretation, administration, and enforcement of the Internal Revenue Code, as well as all other legal matters. Under the IRS Restructuring and Reform Act of 1998, the Chief Counsel reports to both the IRS Commissioner and the Treasury General Counsel.
Attorneys in the Chief Counsel’s Offce serve as lawyers for the IRS. They provide the IRS and taxpayers with guidance on interpreting Federal tax laws correctly, represent the IRS in litigation, and provide all other legal support required to carry out the IRS mission.
Chief Counsel received 95,929 cases and closed 94,323 cases during fscal year 2012. Of the new cases received, and cases closed, the majority related to tax law enforcement and litigation, including Tax Court litigation; collection, bankruptcy, and summons advice and litigation; Appellate Court litigation; criminal tax; and enforcement advice and assistance.
In Fiscal Year 2012, Chief Counsel received 31,295 Tax Court cases involving taxpayers contesting an IRS determination that they owed additional tax. The total amount of tax and penalty in dispute at the end of the fscal year was almost $6.6 billion.


(Source: Internal Revenue Service Data Book, 2012.)