HI snowy Harpenden
Shannon domestic will usually ask if the aircraft is ready and able for the climb to the Crossing cleared level.
Thus all the aircraft are filtered and funneled flow to the entry points in a logical and safe sequence as required by FL, Mno, Track and time.
Re the filed plan showing step climb requirements, this flight plan does not over-ride the ATC clearance which will NEVER include a step climb.
Should the aircraft require the climb for performance / burn / avoidance purposes, then a request via CPDLC or HF is made, ATC look at the big picture, and advise accordingly.
Hope this answers the questions.
glf