The Welsh don't seem to be doing too badly at reestablishing their language and the English went for that big time as well.
I also concur with RGB on this point; Korea is a particularly good example of this, as it happens. The international lingua franca for East Asia, and really, for that matter, large parts of the far East generally was Chinese, owing to China's cultural and economic dominance, but while knowledge of Chinese culture was essentially mandatory for the Korean, and to a slightly lesser extent, Japanese elites, it never came close to displacing the Korean and Japanese languages.
By contrast, where Chinese did displace local languages, there was a very long and consistent period of Chinese rule and settlement. The most obvious example of this is in Southern China, which until the Han Dynasty, was not Chinese at all, but over the course of a few hundred years, was totally Sincizied by a combination of Chinese immigration and settlement, and the displacement thereof of the local languages.
Mind, that's not to say that there weren't other reasons that Chinese displaced local languages. A major factor in it was that before the Chinese settlement, Southern China was extremely lightly populated, whereas Korea and Japan, leaving aside how they experienced no extensive Chinese settlement in their core regions, already had strongly entrenched native populations. However, I dunno how much this applies to Ireland; since the principal reason the above matters is that it leaves the region susceptible to large-scale immigration, which, AFAIK, was not a significant factor in the Anglicization of Ireland.
So why did Anglo-Saxon subsume Norman-French rather than the other way round? I know that it is a resilient language and saw off British (which would become Welsh) and the Viking languages but by those arguments we should be speaking a form of French. Mind you compared to Scandinavian languages, German and Dutch perhaps we do!
What broke the back of the Irish language was the potato famine of the 1840s. It hit the Irish speaking areas hardest.
After the famine Irish speakers made sure their children learned to speak English and it was need to get a job in the British army or civil service and many other state jobs. After this emigration was highest in Irish speaking areas most of these people went to America, England or other place where they need to speak English.
The 20 years after the famine was when most damage was done to the language.
The only way for the language to survive is to have better economic prospects for people who speak Irish in Ireland
[FONT=Verdana, Arial, Helvetica, sans-serif][SIZE=+2]Great Famine rang the death knell for Irish language[/SIZE][/FONT]
http://www.irishidentity.com/extras/heritage/stories/language.htm
Butterfly out Trevalyn/somehow make Trevalyn use measures similar to what he used to fight the Scottish potato famine in Ireland. This can probably cut the famine down to 2 years duration, similar to the situation in the rest of Europe.
Or, perhaps a more creative solution. Instead of settling english-speakers in Ulster, settle them in Scotland and deport Scottish Gaelic speakers to Ireland. A justification for this might be the English deciding to put all those Gaels in one place - why garrison two rebellious areas? This of course would need an earlier POD, and would probably lead to significant Scottish influence on Gaelic... and a much more anglicized Scotland.
Any particular reasons why the English wouldn't do this, anyone?
What might help to explain why Welsh is so vibrant as opposed to the Scots Gaelic and Irish would be that after the 15th century, how many revolts do you see occurring in Wales?