When Ireland’s leaders attended the coronation of King Charles III, it was hailed as a milestone in relations between Dublin and London. The Irish president, Michael D Higgins, the taoiseach, Leo Varadkar, and Sinn Féin’s deputy leader, Michelle O’Neill, set the precedent – their predecessors had shunned previous coronations – to show respect to their neighbour. Varadkar’s partner, Matt Barrett, however, did not get the memo. In the VIP motorcade and in Westminster Abbey, he posted a series of irreverent comments on Instagram to his private group of more than 350 followers. “Holy shit I think I’m accidentally crowned king of England,” he posted from the taoiseach’s car as they approached the abbey on 6 May. The posts, reported in the Irish Times on Saturday, have embarrassed the government and landed Varadkar in a fresh diplomatic blunder. Once inside the abbey, Barrett, a consultant cardiologist, ignored an injunction in the order-of-service booklet to switch off his phone and posted jokes and observations on the ceremony. A paragraph from page 38 in the booklet caught his eye. “The queen’s sceptre and rod are brought from the altar by the Right Rev and Right Hon the Lord Chartres GCVO and the Right Rev Rose Hudson Wilkin CD MBE, Bishop of Dover. The queen touches them in turn,” it said. Barrett posted a photo of it with a green line around the last sentence. “Sounds like the script to a good night out, tbh,” he said. In the list of participants, he noted the Right Rev James Newcome, who has the title Clerk of the Closet. Barrett highlighted this, saying: “Had this job until my early 20s.” Later he posted a photograph of Charles wearing his crown and compared it to the sorting hat in the Harry Potter books. “Was genuinely half expecting it to shout ‘GRYFFINDOR,’” he wrote. Some commentators said Varadkar, who caused a flap in March by joking about Bill Clinton’s affair with Monica Lewinsky, had not added to the Irish delegation’s dignity by being filmed picking his nose. “Their morning went from Hankiegate to Instagate,” said the Irish Times columnist Miriam Lord. Barrett’s interventions undercut the attempt to apply post-Brexit balm to diplomatic relations, she said. “Strange behaviour from someone who is there as the guest of the person representing Ireland at this important state occasion and whose presence is seen as a symbolic gesture marking this state’s improved relationship with Britain.” The taoiseach’s office made no immediate response. Paul Costelloe, an Irish designer with links to the royal family, told the Sunday Independent the comments were insulting to Britain. “You can say it’s amusing but it’s just terrible,” he said. “Obviously, Matthew thought it was a great joke.” Costelloe urged Barrett to apologise and said Varadkar should have told his partner to turn off his phone. The British now had material to say the Irish did not know how to behave in such circumstances, he said. “I just hope the English press don’t get hold of it.”