How do active irregulars go back underground?
US Train Operation (3.2.1) combined with voluntary removal (1.4.1), or Reset Phase (6.6).
Does transport allow active irregular to be placed underground at new space?
No. Transport may not do anything whatsoever with US pieces.
Does transport of active ranger put it underground in new space?
Transport flips ALL Active Rangers on the map Underground, regardless of whether they moved or not.