not quite a mandolin story, but his tenor is tuned the same way and it is reputed that he asked his parents for a mandolin...and ended up with a banjo! I just stumbled across a link to an RTE series co-presented by Barney called the Green Linnet. All 6 episodes avaialble to view here http://www.rte.ie/player/#!v=1137908
RIP Barney. That was an interesting watch, I viewed the first episode and then the Green Linnet Remembered, filmed in 2006: http://www.rte.ie/player/#!v=1137928 Which included a bit of Mandolin, when they visited Italy.
RIP Barney. Ar Dheis De a Anam.
