When Afro Celt Sound System burst onto the music scene some 15 years ago their impact was so instant, so astounding, that it hit like a thunder crack. Here was a band unlike any other, a band whose fusion of West African rhythms, Irish traditional music and cutting-edge dance grooves battered the senses and unleashed a wellspring of joy and liberation. When they added diverse new touches – Indian bhangra, Arabic influences, dub reggae and more – they did so seamlessly, in ways that only enhanced their sound and emphasised their openness. A supergroup whose line-up expanded and evolved around four core members (Simon Emmerson, James McNally, Iarla O’Lionaird, Martin Russell), the Afro Celts’ pan-global sound redefined dance music and stumped music critics. They remain defiantly, enigmatically uncategorisable.