I have become more and more convinced—and it is only a question of driving this conviction home to the English working class — that it can never do anything decisive here in England until it separates its policy with regard to Ireland most definitely from the policy of the ruling classes, until it not only makes common cause with the Irish but even takes the initiative in dissolving the Union established in 1801 and replacing it by a free federal relationship. And this must be done, not as a matter of sympathy with Ireland but as a demand made in the interests of the English proletariat. If not, the English people will remain tied to the leading-strings of the ruling classes, because it will have to join with them in a common front against Ireland. Every one of its movements in England itself is crippled by the strife with the Irish, who form a very important section of the working class in England. The primary condition of emancipation here—the overthrow of the English landed oligarchy—remains impossible because its position here cannot be stormed so long as it maintains its strongly entrenched outposts in Ireland. But, once affairs are in the hands of the Irish people itself, once it is made its own legislator and ruler, once it becomes autonomous, the abolition there of the landed aristocracy (to a large extent the same persons as the English landlords) will be infinitely easier than here, because in Ireland it is not merely a simple economic question but at the same time a national question, for the landlords there are not, like those in England, the traditional dignitaries and representatives of the nation, but its mortally hated oppressors. And not only does England’s internal social development remain crippled by her present relations with Ireland; but also her foreign policy, and in particular her policy with regard to Russia and the United States of America. —Marx, Marx to Ludwig Kugelmann In Hanover
In other words, for the British workers to liberate themselves, they must fight for the Irish workers and support them in both words and deeds. Marx advocated replacing the U.K. with a voluntary federation of nations, quite akin to the U.S.S.R.
Do you have any source to support your claim he committed genocide?