Celts were not "native" to Ireland. Celts originated along the same migration routes as the Germanics and Nodics. The "Celts" were not one distinct tribe, but in fact many tribes that ranged far and wide ove much of Norther Europe from modern day Switzerland to Spain aka Iberia/Gaul on up to the British Isles and Ireland.
The Helvetians from what is modern day Switzerland were and are Celts. The Helvetii were a Gallic tribe or tribal confederation occupying most of the Swiss plateau at the time of their contact with the Roman Republic in the 1st century BC. According to Julius Caesar, the Helvetians were divided into four subgroups or pagi.
The Boii were Celts. This tribe ranged from Cisalpine Gaul (northern Italy), Pannonia (Hungary and its western neighbours), parts of Bavaria, in and around Bohemia (after whom the region is named in most languages; comprising the bulk of the Czech Republic), and Gallia Narbonensis. In addition the archaeological evidence indicates that in the 2nd century BC Celts expanded from Bohemia through the Kłodzko Valley into Silesia, now part of Poland and the Czech Republic.
There were many more as well. More Celtic peoples lived outside Ireland than in it.
