If one was to set out to write an exhaustive list of the grievances that have historically divided the English and the Irish, one would not do so in a short article for a news website. You’d need to write a book. Perhaps several.
It might suffice to say that the English and the Irish have been doing terrible things to each other for the guts of a millennium, starting with the Norman invasion of Ireland and running to the Warrington bombing, with famines, plantations, pogroms, and individual murders of immense cruelty in between.