You would have to check availability but, in general, it's better to do the long flight TATL and have just a very short flight rather than do two middling length flights. This is principally because it allows more time for sleep (especially eastbound) but also because both the service and the seating are much better on the TATL.
For *A this would suggest DEN-FRA-DUB and for OW DEN-LHR-DUB. Can't speak to ST. But ultimately you will be led by availability.