Answer: Republican party (especially in rural states like Wyoming, Colorado, Idaho or in the so-called Deep South) is fundamentalist attached too much to the original myth of the USA (17th century British/English immigrants) which means that it is traditionally very hostile to Catholicism. That is the reason why Irish (in spite of speaking English language) are not always welcomed and always suffered prejudice in England.
Explanation: Republican hostility towards Catholicism (Irishness) was quite visible during electoral presidential campaign Kennedy (Democrat) x Nixon (Republican). Democratic party is more inclusive than Republican party. It means it is more open to globalization, other cultures and their values.