Most are Irish, but "Travellers" are a particular separate group and culture within the Irish ethnic group that are distinct from the mainstream Irish population because of many centuries of isolation. The reason many go to the UK rather than stay in Ireland is because I believe Irish law and police are much tougher on them and Irish people generally are hard on them, they find the UK much easier pickings basically. They are not all violent (despite stereotypes), and they are not criminal groups who live on land that doesn't belong to them at all. And no, they don't threaten and attack those who attempt to evict them, nor make their money from scamming and stealing from elderly and vulnerable people.