My post was about the chances of the local native language to survive, not just the numbers of people in Ireland or the eastern part of Sweden in itself (as they would be in part made of English and Swedish speakers, respectively, as well as of those who speak Irish and Finnish). In 1700, native Finnish speakers (Finnish as first language) made up 82,5% of the people in the eastern provinces, and c. 86% in 1842. Comparatively, how big was the proportion of native Irish Gaelic speakers in Ireland in those years?
I think you are partly making my argument for me. Historically, in comparison to the English in Ireland, the Swedish had been less willing to use force in Finland to assimilate the Finns, and they also had fewer leverages (and fewer overt reasons/excuses for repression) in Finland, due to the similarity in faith and less political issues separating the ethno-linguistic groups all around. Finns generally had the same rights as Swedes did under the crown, and many of the nobles, bureaucrats, members of the clergy, etc, in Finland did come from originally Finnish speaking families - the one major difference between the rights of Finns and Swedes was that the Finnish language did not enjoy an official position in the same way as Swedish did, and was not seen as a "civilized language" of governance, culture and science. We know that the Finnish language-national movement got already started with the Fennophiles of the late 18th century and then with the Fennomans in the early 19th century. When we get towards the end of the 19th century, there definitely would be a growing movement demanding linguistic and cultural rights to the Finns, even if it might not demand separate political rights (in other words, separate Finnish representation) yet. The fact that it will be here happening in the context of Swedish rule instead of Russian rule makes the movement different, but it would not pre-empt it. You don't need the level of conflict and assimilation Ireland experienced to see Finnish nationalism to be born ITTL.
All the time while this is happening, Russia would be growing in power in comparison to Sweden, and Stockholm will need to take this into account in its policies in Finland. In Britain, while there was a long-term rivalry with the French, and the French supported Irish nationalism, France was never as strong in comparison to Britain as Russia will be by the 20th century in comparison to a Sweden that still holds (most of) Finland. The Swedish leadership and elite will then need to compromise more than the English did in Ireland, lest the Russians manage to convince the Finns that they can offer a better deal than Stockholm can.