In the 1800s what was a major reason why Irish resented British in Ireland?

The main factor was the Penal Laws. These were laws that discriminated against Irish Catholics, placing significant social, economic, and political restrictions upon them. For instance, they were not permitted to own land or hold public office. Furthermore, under these laws, the Catholic Church was suppressed and its practices curtailed. These laws were primarily enforced under the guise of preventing Catholics from gaining power and to bolster Protestant dominance in Ireland.