Its in New Quay, not Fishguard! The NLW photo reproduced in, amongst other books, G.W.Hall 'Old Metal Mines of Mid Wales', shows a quarry apparently some way from the main workings. The first ed OS 25" map shows nothing at all as I recall. I think there was a useful note in an early WMS newsletter - advise checking the online index and then the relevant page in the online scans of the newsletters.
Essentially though - no kinown underground access, and the very scantiest of surface traces, to the best of my knowledge.