Lack of compatible bass notes on a B/C box really doesn't matter! Many Irish players play the 'wrong' basses on semitone tuned instruments - it's only percussion anyway so if you don't overdo it, nobody will notice, especially if your right hand is up to scratch!
If you must have compatible chords, why not play Irish tunes on a GD box- most tunes are quite possible, as well as many of the rolls & runs inherent to Irish music, especially if you have a customized box to suit your requirements.
Even more so if you have a 2 and a half row, with a few extra notes of your own choice.
It's still more limited than a chromatic box, but the built in bounce of the melodeon style more than compensates, I think?
Anyway, great Irish music can be played on a ten key, one-row melodeon with NO extra notes - go and listen to PJ Hernon or Johnny Connolly.
Again- about 'Irish' bass playing - in 1965 I had the same puzzlement about how it worked as Dazbo in 2004 above, so I asked a very competent box player to explain the left hand on his two row box.
Tim Lyons did just that- he said 'I don't know, I look the other way'- a method I have used for over 50 years now....