I don't believe security clearnances would or even should be required at other gov't agencies such as the IRS, HHS, etc.
As a general rule, people working at the IRS shouldnt/wouldnt require clearances.. but there are exceptions...
For example, IRS agents assigned to certain federal law enforcement task forces would likely require clearances (investigating trans national narco trafficking, international human trafficking, etc.)..
While I hate the IRS as much as the next guy.. from a law enforcement perspective, agents from some of the non traditional law enforcement agencies often bring HUGE value to various task force groups..
For example, we had a US Postal Inspector assigned to a US Marshals led task force that I was once assigned to, that dealt with finding very bad people that were very good at not being found..
The postal inspector had jurisdictional authorities that others in the group didnt have.. and more importantly, he had a specific set of skills and experience that no one else in the group had.. he was critical in not only helping us find some people, but also in levying certain charges against those same people that we otherwise wouldnt have pursued (or likely have known to pursue)..
because of the national security threat that some of the people we were tracking down posed and because we required access to certain federal information and systems that required certain levels of clearance, most everyone on the task force carried a security clearance..
there would be a handful of others outside the "agents" group within the IRS that I can think of that likely hold clearances as well..
80% +/- of all clearances are DOD though.. DOJ, DHS, DOS, DOE, etc..etc.. all combined only hold roughly 20% of current clearances..
Something that is also important to understand about clearances is that different agencies have different clearances and different processes for awarding and maintaining those clearances as well..
For example... a DOD Top Secret is not the same thing as a DOS Top Secret.. which is absolutely different from a Dept of Energy "Q" clearance (their "top secret" is called a "Q")..
and while the DOD and DOS generally "allow" each others clearances.. for example.. If I hold a DOD TS SCI.. and I have a need to be read in on a DOS SCI program... DOS typically recognizes the DOD clearance and allows me to move forward with the read in procedures..
that is NOT true however with DOE.. they see their Q clearance as a different standard.. and will not automatically allow a DOD TS TSCI holder to be read in on equivalent level material... there is an entire process for having your DOD clearance reviewed by DOE and additional steps that must be gone through before you "see" their stuff...