As with any employment, the relationship between the organization and the employee should be protected from the public. If there are criminal charges to investigate, then the federal or state law enforcement should be involved. Employees can be vulnerable to public outrage and we are all due a fair investigation for any wrong-doing.
Unless there is a criminal investigation that requires employee information to be handed over to legal authorities, the confidentiality any employee information of any organization should be protected indefinitely. Arguably even more so in an organization like the IRS where employees just doing their job could be put in danger because somebody is pissed off with their tax return. Employees in debt collection agencies don't give out their personal information for the same reason.
I think that the argument that because taxpayers pay public sector salaries, they should also have access to public employees' information is a poor argument. I pay the wages of the employees of private companies whenever I purchase something, but that doesn't give me the right to walk into Wal-Mart and demand to see everyone's T4 and home address.
Understanding the reasons why information should be protected is the key here. If you have a former employee who then has issues on a new job, I think the information from a former employer could be allowed that may help (or possibly hurt) an employee. As far as the IRS is concerned, they can't protect just their own, as this question is stated, to protect their former employee doesn't seem correct. Working for the IRS is working for an employer.
If an employee has left the IRS and their information is needed in a court or law situation, then that information should not be protected. No one's should. If it is general information for the general public, I don't understand why the public should know anyone's corporate information or government information.
People who take public sector jobs in the government are giving up part of their right to privacy by agreeing to be paid by taxpayers rather than by private employers. Once you agree to that, you are open to issues of the public trust, and if that mean handing out information on employees so be it.