"Clohessy, do you know what you are an omadhaun. Do you know what an omadhaun is?
I don't sir.
It's the Irish. Clohessy, your native tongue, Clohessy. An omadhaun is a fool."(p.155)

All of Franks life, him and other boys his age are being put down and told they are fools useless.
Alcoholism is usually brought about by a feeling of uselessness and a lifetime of being put down by and authorative figure.
This seems to me like the older Irish generation fuels the younger Irish into becoming alcoholics like themselves and this vicous cycle is really the downfall of the Irish community.