What makes some languages survive and others fade away? The oldest language in Britain, Welsh, is thriving, but across the Irish sea Gaelic is barely spoken by 1% of the population on a daily basis (though at least 10% claim to be fluent in it). And this is despite the Irish Republic spending a fortune since 1922 in support of Gaelic.
To answer the question you have to go back to the Protestant reformation. A key argument of Protestantism was that you should be able to read the bible for yourself, in your own language, so you could make your own connection with God. But the Catholic church argued that it was evil for the Bible to be in anything but Latin (despite the original New Testament being Greek), and that it required priests to interpret the word of God.
The Catholic position was about power and control. They knew that most of their congregation couldn't read, and the ones that could read their own language were not schooled in Latin. So they had no way of telling whether what the priests were saying was true or not. If the church said that selling indulgences was allowed by the Bible then there wasn't anyone to contradict them.
The Protestant revolution was sparked by a German monk, Martin Luther, reading the Bible for himself after painfully learning Latin, and discovering that much of what the church was saying simply wasn't in the bible at all. He nailed his protest to the door of the church (in German) and a revolution took off.
The race was on to translate the Bible into various languages, and the Church was equally determined to prosecute anyone who translated the Bible for heresy. William Tyndale, the man who translated the Bible into English (and on whose translation the King James Bible would be based) was hung, drawn and quartered for heresy.
In the meanwhile Henry VIII was having a tortured love life, and decided to divorce his wife Catherine and marry Anne Boleyn. He had hoped the church would annul his first marriage as Catherine of Aragon had been the widow of his brother. But the church, lobbied by Catherine's powerful royal family in Spain, refused.
Anne Boleyn was a Protestant, and she suggested a solution to the love-sick Henry. Why not reform the church and make himself the head? Henry agreed this was a good idea, though his idea of reform was to keep everything the same but just exchange the Pope for himself as head. So England switched religion. Anne alas gave birth to a daughter Elizabeth and not the son Henry desperately wanted, and Henry, by this time in love with Jane Seymour and sure that she would give him a male heir, beheaded Anne for treason.
Jane Seymour died in childbirth, but she was a committed Protestant and her son Edward VI undertook a full blown Protestant revolution when he became king. His idea was to make English the official language of both church and state for the whole of England and Wales. It looked like Welsh would die.
Edward VI died of tuberculosis at just 15 years old. After a brief reign by his elder sister Mary (who tried to restore the Catholic faith and the Latin mass) he was succeeded by his sister Elizabeth, daughter of Anne Boleyn who had started the entire revolution.
Elizabeth is the key
Elizabeth was moderate in all things. She wanted to make it easy for people to obey the law and hated the idea of picking fights with her own people. She couldn't keep Mary's Catholicism because the issue of Mary's martyrs was now a sore point with the Protestant part of the nation. But she thought Edward VI's ideas were too extreme for people to bear. So she invented the Church of England as it is today. She kept some of the catholic-style ceremonies that the ordinary people were so attached to, but she wanted all services to be held in English as a nod to the Protestants.
And she made a special settlement for Wales. Church services in Wales, she declared, should be conducted in Welsh, and she ordered the Bible to be translated into Welsh.
The Welsh people were overjoyed. Previously their church services were either in Latin or in English, neither of which they understood, but now they were in their own vernacular, complete with a genuinely fine translation of the Bible that they could read. A Welsh Bible was placed in every church and as soon as a family earned enough money, they learned to read and bought one for the home too, to record family events and for private worship.
Just as Elizabeth's settlement in England created an upsurge in literary activity in English, her settlement in Wales created an upsurge in literary activity in Welsh. Welsh was now both a liturgical and literary language.
Elizabeth's Welsh settlement was so well received that she tried to do the same thing in Ireland. She offered to set up the Church of Ireland and have the bible translated into Gaelic too. But she got rebuffed by the staunchly catholic Irish. They wanted the old ways in Latin, even if they couldn't understand any of it. She went ahead anyway and set up a Church of Ireland, but it had only a small congregation, and she died before she could organise the translation of the bible into Gaelic.
Fast forward to the modern world
As the centuries rolled on, and Britain developed the industrial revolution and acquired an Empire, English got more and more dominant as the language of the state, of Empire and of commerce. There was a big push to get all schooling in the whole of the UK done in English to equip people for the job market as commerce was now entirely in English.
But in Wales, Welsh reigned supreme in one place - the church. No one dared to interfere with Good Queen Bess's decree that services in Wales should be in Welsh. By the 19th century Elizabeth was revered as one of the greatest rulers of Britain, so no one wanted to interfere with her legacy. So in Wales, Welsh kept getting spoken on Sundays. It wasn't till the 1960's when people stopped going to church did Welsh start to decline. But because the decline was not severe, it was possible to keep the language going, especially when the Thatcher govt agreed to allow Channel 4 to be broadcast in Welsh in Wales.
In Ireland it was a different story. Despite becoming independent in 1922 and spending money to revive Gaelic, there was simply no space it owned. The Catholic church stubbornly continued to conduct church services in Latin right up to the 1970's and when they switched language, they switched to English. Commerce too was conducted in English as there was now a new English speaking superpower (the USA) spreading the language around the world.
Welsh survived because Elizabeth I created a space (the church) where it reigned supreme. Gaelic died because the Irish rejected her attempts to create a similar sacred space for Gaelic.