https://www.marxists.org/archive/marx/works/1870/letters/70_04_09.htm

“And most important of all! Every industrial and commercial centre in England now possesses a working class divided into two hostile camps, English proletarians and Irish proletarians. The ordinary English worker hates the Irish worker as a competitor who lowers his standard of life. In relation to the Irish worker he regards himself as a member of the ruling nation and consequently he becomes a tool of the English aristocrats and capitalists against Ireland, thus strengthening their domination over himself. He cherishes religious, social, and national prejudices against the Irish worker… This antagonism is the secret of the impotence of the English working class, despite its organization. It is the secret by which the capitalist class maintains its power. And the latter is quite aware of this.”