US senators and tax law professors have accused Apple of multi-billion-dollar tax avoidance – shifting billions of dollars of profits to Ireland where Apple has found a way to pay little or no tax. Apple boss Tim Cook disagrees. He insists there is no shifting going on, and that profits earned by Apple's Irish operations are down to the hard work and expertise of almost 4,000 staff in Cork. The Guardian went to the Emerald Isle to test these competing claims