I have just come across this post having spent hours on our visit to the Isle of Man combing the graveyard at St Peter's Onchan for the grave of my husband's maternal grandparents! Frederick John McCarthy died at Noble Hospital in Douglas in 1944 but the Ancestry transcript said he was buried in Onchan. Thank you both for having asked the question and given the answer that the cemetery concerned would be at Douglas!
Unfortunately, we only have a day left on the island, so are unlikely to be able to track down the grave in the time available.
But at least I now know where to look!!