yes, the IRS has a union, although membership is not required...the union does actively recruit (pressure?) employees to join; but if one wants to advance into management, membership is seen by some to be a bad choice.
the compensation package, particularly benefits, while good is not as nice as that for LA County employees.
personally, i think where there is a single buyer of labor (in this case the government) a single seller (union) is needed to level the bargaining power.