When the IRS enters your life, look for a tax attorney that truly specializes in tax controversy. Many tax lawyers hold themselves out as qualified to handle an IRS dispute, perhaps because it is an area of practice in which they one day hope to become proficient. That is not what you want.
Most tax lawyers do not specialize in handling tax controversies. So you do not want tax lawyer, per se. When you are looking for a lawyer that specializes in tax controversy, find one with a lot of experience handling tax controversies of all sizes. Even if you are referred to a tax lawyer by someone else, find out how much experience they have, how many cases in tax court they have handled, the qualifications of other lawyers in the firm, and look closely to see if they are a real pro, or someone who is trying to become one.
There is no substitute in this area of practice for having a lot of experience doing it. As for credentials, good tax controversy specialists may have a lot of credentials beyond their jurisdictional doctorate degree, and others equally good may simply have their J.D. degree. Some are also CPAs. Some have LL.M. degrees in taxation. Take care not to get sold on the idea of extra degrees and credentials over true serious experience in handling tax controversies.