Van Der Merwe did finally get the try Canada's efforts deserved on the breakaway in the 68th minute after snaffling up Irish centre Jared Payne's chip kick.
By that stage, though, replacement hooker Sean Cronin had scored Ireland's fifth try and there was still time for fullback Rob Kearney to match his brother's tally after a length-of-the field sprint from winger Keith Earls.
Payne scored the seventh try five minutes from time after a break from Sexton's replacement Ian Madigan.
Canada next face Italy in Leeds, while Ireland take on Romania and the Italians before returning to Cardiff on Oct. 11 for what is expected to be the pool decider against France.