You say, “I thought the Irish spoke English?” They do.
But it is not the original language of the Celtic peoples of the British Isles as depicted on the map: Irish (Green), Scots(Blue), Manx (the tiny dot of Orange in the Irish Sea), Welsh(Red), Cornish(Yellow), and Breton, France(Black). The original languages of the original people who settled in the British Isles were Gaelic or nowadays called Goldielic (Irish, Scots Gaelic, Manx) and Brittonic (Welsh,Cornish,Breton) languages. How this came about is an extremely long and complicated subject, but I’ll try to give a brief summary.
According to Nora Chadwick, the Celts were the first people to settle in the British Isles, in about 800 BC. The Celtic civilization emerged as dominant in Europe in about 500 B.C. At its height, the Celtic civilization stretched from Northwestern Europe to Turkey. Most of what we know about them comes from archeological digs and what their enemies wrote about them. Literacy did not come to the Celts until Christianity arrived. From the time of Christ until 1066 A.D. The Celts of Britain were under siege from the Romans, the Saxons and other Germanic tribes, the Danes, the Vikings, and finally the Normans. Because the Celts were tribal, it was easy to play one tribe off against the others.
The Celts of Ireland were luckier in the beginning. They were left alone during the Roman-Celtic and Saxon-Celtic wars in England. Then, their luck ran out. The Vikings were the first invaders beginning in 800 A.D. Then came the Normans in 1166 AD followed by English King Henry II in 1171. Then the English institutions of the invaders declined until Henry VIII sent troops to reconquer. The Celtic Irish went through all kinds of hardship, even slavery and starvation in their long sad history. In the 1600’s they reached the low point politically with Cromwell’s Reconquest and the defeat at the Battle of Boyne. The native Irish were pushed out of the fertile lands to the windswept marginal lands of the West. This was the area where the Gaelic language survived.
After the terrible potato blight famine of 1845-1849, the population of Ireland declined from 8 to 4 million, much of it through emigration to America. The Gaelic language usage declined significantly. Many Irish politicians, supporters of independence, preferred to continue requiring English instructions in the schools.
But, they were never completely defeated. Nor did they convert to Protestantism despite the establishment of the Anglican church as the Church of Ireland. In the end, Irish independence from Britain was long delayed but not denied.
Recently, there have been efforts to revive Irish Gaelic and all of the other Gaelic languages. Undoubtedly, Benny Lewis cannot call himself the Irish Polyglot without being able to speak Irish Gaelic, the original language of the Irish people.
The first wave of Britons to emigrate from Wales came with the Roman Army. When the Roman Army left Brittany, the mercenary Briton soldiers remained
Cornwall became separated from Wales in 577 AD in the aftermath of the Battle of Deorham. The Saxons defeated the Britons and killed three Briton kings. The Celts were Isolated and surrounded by the Saxons. After loses at the Battle of Deorham, a second wave of Britons came to Brittany thought to have escaped the Anglo Saxons from the warfare in Wales in the 6th Century. From these settlers evolved the Breton language in Brittany from 9th Century.