Tommy Bowe and Augusto Petrilli then exchanged scores before Paul Marshall went over to set up Trimble's late show as the hosts claimed a 25-24 win. Humphreys kicked 10 points for Ulster while Morgan Parra booted two conversions for the French side.
Cardiff Blues stayed firmly on course for the European Cup quarter-finals after subduing Harlequins' spirited resistance with a 23-12 win at a windswept Arms Park.
A Ben Blair penalty and a try from Dafydd Hewitt converted by Blair put Cardiff in control before Haldane Luscombe went over and Chris Malone converted to close the gap.
But two Blair penalties and and a converted Shanklin try put the match beyond doubt before Mike Brown grabbed a late consolation.