This appears to be made of bronze or brass and is about 19mm in diameter. I can't find any reference to it on the internet. Any ideas?

Is it a French 20 Centimes Token (Jeton). I suppose is it from 1920's, but is hard to say more... in that time were hundreds or thousands types due to lack of small coins after WW I...

But why would you have a harp at the reverse? I thought that's very typical of Irish coins. Maybe a dual currency for both countries? Or an island that took both currencies?

I don't think so... The tokens designers were very imaginative in their work... many tokens are real pieces of art (you can say cheap art thinking at the low value of these tokens), and maybe because the tokens were also advertising...
Maybe into the 50 C-mes token was a shoe... No kidding, I have a German notmunze from Pirmasens city with a lady shoe on Rv!

Perhaps an Irish establishment showing off his French?

That makes sense since Pirmasens was and (and to some extent is) a center of the shoe/leather industry. They also have a shoe museum there ...

As for the harp, that is not necessarily a reference to Ireland. Here are quite few images of similar "jetons à consommer" (consumption tokens): http://www.jetons-monnaie.net/p/jetonaconsommer.html Have not spotted the harp there though.

Christian

It is one of those tookens saying
a consomer
which means like
" good for one drink"

It is not amongst the common examples

http://www.jetons-monnaie.net/aindex.html