"Traveller" is a European-English spelling. In American English, one who travels is a "traveler." The Travellers' name for themselves is "Pavee."
The Irish Travellers are a nomadic minority still maintaining a mostly-separate culture on the island. The Travellers constitute less than a half-percent of the population, with 25,000 in the Republic and 1,500 in the North.
The Travellers are indeed nomadic. These days, their normal mode of transport and habitation is a travel trailer or recreational vehicle. Some have homes but even of those who do, some do not live in houses, in a normal sense. Instead, they travel. For at least part of the year, many Travellers are mobile. You'll see the Irish Travellers, throughout the land, at "halting sites" along sides of roads, where they tend to stop in groups.
The conflict between a settled population and an itinerant one is obvious, people being how they are. The difficulties between the majority Irish and the Travellers has likely been as much a conflict of ideas about property as it has about class and race.
Race, indeed, does not factor into the standard Irish thinking about the Travellers many reject the idea that their ethnicity is distinct. Most Irish people, though, could usually recognize a Traveller in an instant, regardless of attire or circumstance.
The popular name for the Travellers amongst the "settled" people used to be "Tinkers." This term is somewhat belittling, and it has fallen out of use. Now, most settled Irish casually refer to Travellers as "Knackers," which is utterly dismissive and racist. (Many Irish would take exception to this opinion, not only because "I'm not racist" but also because "Travellers are not a race.")
There is linguistic evidence that Irish Travellers predate many of the recent immigrations....
The word Tinker may (or may not) be from the Irish (or old English [I haven't sorted out the apocrypha from the science on this one]) words for "tinsmith." What is clear is that the Travellers have always practiced business that lent itself well to mobility, including tinsmithing repairing pots and other household metalware. That carry-on, as well as much of the door-to-door sales that was also profitable, went the way of old technology when plastic and other modernizations displaced that sort of trade. This is true also of horse-trading, in which Travellers were heavily involved, before the broad use of automobiles and trucks.
These days Travellers are involved in other forms of mobile-friendly commerce scrap dealing, buying and selling of goods. I've been offered carpet for sale a stereotypical Traveller commodity. The laying of tarmacadam, or asphalt, in the "settled-Irish" driveway, is also a business that many people typically associate with Travellers.
The racism leveled at Travellers is profound. The conflict between settled people and Travellers is obvious, when you consider that the Travellers have an entirely different conception of property. And, as Ireland modernizes, the differences between these two separate cultures only becomes more exaggerated.