There is no legal requirement for Congress to create the IRS. Congress created the US Dept of the Treasury (on Sept. 2, 1789) and a Secretary of the Treasury. Oddly enough, all the tax laws say that
the Secretary of the Treasury will administer/apply/enforce the tax laws, there is scarcely a law anywhere that says that the IRS
by name will do anything. If the Secretary of the Treasury wants to create a division within his department, merge or split or rename existing divisions, the better to discharge his legal responsibilities, that's within his authority and Congress does not get to micromanage these cosmetic changes. Arguably the Secretary of the Treasury could, this very day, reassign the responsibilities of the tax laws to a newly formed subdivision of the Treasury which he could name the Mickey Mouse Club and it would be legal.
On June 30, 1864, Congress did create the position of Commissioner of Internal Revenue, to be appointed by the Prez with the advice & consent of the Senate. The existing of a Commissioner implies the existence of a commission for him to supervise.