How are Winston and Julia betrayed? Explain the various ironies involved in this scene.

Winston and Julia betray themselves in multiple ways. For example, unbeknownst to him, the Thought Police read Winston's journal. Government cameras and microphones also record Julia and Winston making love amid the bluebells in what Winston calls the Golden Country. Winston and Julia are recorded as well at O'Brien's agreeing to help to overthrow the regime. But if you are talking about the scene of their arrest, it is in the room above Mr. Charrington's shop.
This is ironic because it is the place Winston had most associated with safety from government surveillance. It never occurred to him that Mr. Charrington could be with the Thought Police. Winston thought he had found a haven. In fact, he likened the room to the coral paperweight that he had bought from Mr. Charrington. Winston liked to imagine the room as encased in thick, wavy glass similar to the glass protecting the delicate piece of coral. But ironically, the room is just as fragile as the paperweight the Thought Police shatter.
Another irony is that Julia had been talking about taking down and cleaning the picture that hid the video camera filming them, but never did, so they never knew it was there. They thought they were experiencing privacy, but all the time they were being watched.
They also missed the danger in the clock being set to the wrong time. Winston sees it, but ignores it. If he had been alert to danger, that strange fact might have caused Julia and him to flee. But, ironically, he is too sure they are safe there.