You should be just as confident that animals are having experiences as you are that your fellow human beings are. They TELL you that they are having experiences. Have you never known an non-human mammal in your life?
If you were emotionally motivated to think of Irish people as not having the same full experience of life and suffering that you do (perhaps they taste good, or perhaps you have a coal mine their children labour in) you will find that you can convince yourself that they don’t. You are engaging in a set of obvious psychological defense mechanisms to protect your worldview that lacks any coherent ethical structure against ideas that are ethically consistent.