In what context or time period? Anglo-Irish history is extremely complex and lengthy, and the relations between them varied greatly over time.

If you are asking about 19th century relations, I can shed some light on the topic, though I'll need more to go on than this.

This is a very broad question, I would suggest you re-submit it with a more narrow period of time, or find what you need on the FAQ. For instance, you could ask about the British Colonization of Ireland and the impact that had on later relations, or the Famine, or any number of periods.

This question is an example gross generalisation, stereotyping and racism. The assertion that "the British" were cruel towards the Irish is an example of the acceptance of nationalist myth as History. That relations between the two nations were often poor is a reasonable statement. That, at times, cruelties were inflicted by some Brits is a fair comment. That British troops carried out what would be described now as war crimes whilst fighting Irish guerillas is also a fair comment. That Cromwell's troops massacred the (English Royalist) garrisons of Drogheda and Wexford is fair comment. However, to seemingly accept, as the submitter does, that "the British" were cruel to the Irish, as an apparently objective truth, is, at best, to be a simplistic acceptance of myth as history. One could similarly ask "Why were the Americans so cruel to the North Vietnamese?", or "Why were the Americans so cruel to Africans?", or "Why were the French so cruel to the Algerians?", or indeed, "Why were the Irish so Cruel to the British?".