Most are Irish, but "Travellers" are a particular separate group and culture within the Irish ethnic group that are genetically distinct from the mainstream Irish population because of many centuries of (self) 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 more clued up about them, they find the UK much easier pickings basically. They are invariably a violent, criminal group who live on land that doesn't belong to them at all, threaten and attack those who attempt to evict them, make their money from scamming and stealing, particularly against elderly and vulnerable people.