Unknown's withdrawn from Irish part in series that comes to nought? (9)

I haven't cracked this one yet — but the brilliant team at Times for the Times have:

Take the “unknown” Y out of COUNTY DOWN, which gives me the irresistible excuse to point you to the wonderful Ballad of County Down from the Hoffnung series of concerts spearing the early American sense of inferiority in the Space Race. Good music too.