In case you are dealing with an IRS case, whether they have already filed one against you or are just about to look into it, it is recommended that you seek the professional legal help from an IRS tax lawyer.
The reasons are fairly obvious. Tax matters are basically legal matters, so any problem arising out of it are better off being explained and dealt with by someone knowledgeable on the subject matter--barring any computation issues, of course.
Delayed payments, fines and penalties, potential criminal liabilities--all of these are what will most likely be better dealt with by an IRS tax attorney.
Getting Legal Help
The good thing about handing this over to a lawyer is that they are professionals. They know the laws inside and out, and although it may never be black and white, they can anticipate and deal with the issues that may arise from it.
Having the IRS hounding you for one thing or another is bound to be very stressful. The last thing you would want to deal with is to have the matters become even more complicated because you misunderstood an instruction or a particular context of the discussion. Having a lawyer by your side during this process can help you better understand what is going on, thereby letting you keep a clear head so you can decide what next step you should take.