Inveterate collaborators, the Chieftains celebrate their 50th anniversary with a new generation of roots acts. That most are drawn from Americana circles allows Paddy Moloney et al to emphasise folk's transatlantic trade, with Bon Iver lamenting ethereally on "Down in the Willow Garden", an Appalachian murder ballad of Irish descent. The Punch Brothers mix bluegrass mandolins with tin whistles, the Carolina Chocolate Drops make the link between hoedown and jig, and the Civil Wars' harmonies fit snugly with harp and pipes. There are occasional flat spots (Paolo Nutini, the Secret Sisters) but ease, exuberance and quality easily outweigh any faux moments.