Because the vast majority of the votes on the show come from girls aged 10-17. Girls aged 10-17 LOVED Eoghan and voted for him based on his looks and didn't have a grasp on what real talent and decent singing actually meant. Therefore, his looks and "cuteness" took him through so far in the competition. How else do you think the medicore George Sampson win BGT?
Number One's in Ireland mean sweet fuck all if it doesn't match the UK charts. He only managed 14th position in the chart with his debut album, which, granted, for a new artist is pretty decent, but for someone getting so far in the competition that always produces number one singles and albums from the finalists that release material, it's pretty poor. His album lasted only 8 weeks in the Irish chart and total sales were only just over 30k so I have no idea where you got platinum from.