To start things off, Salide has an (unofficial) land area of about ~91,250 sqkm. Population is ~22,617,000. This gives a population density of 243/sqkm. The UK, in comparison, has 270/sqkm for GB and NI, and Ireland (Republic) has 70/sqkm.
The terrain map is incomplete. The topography you see is where major features are located, but I have an issue with scaling mountains and valleys, so it does seem like it has a very aggressive terrain. Most of the land currently on the map does not go above 100 metres elevation, so can still be considered quite mild. In fact, Salide is fairly flat with dividing ranges interspersed somewhat like you see on the map. (Take a topo map of the UK and tell me how there are 67 million people living there
The population is indeed concentrated towards the southeast, with the highest densities between Seiskin, Cappach, and Dunmuir. This area is several times larger than the London metropolitan area (14.25 million) and will be home to a significant amount of the population. Initial estimates were between 11-14 million for this area and spread out over much more land (to the point where it wouldn't be considered one single metropolitan area, but two or three.) Among the other large cities, I estimated that each would have a metropolitan area up to or exceeding 1 million. Add to that, the numerous small villages and towns that will dot the areas in between, as is seen in the UK, there should be no problems with the population.
On the issue of farmland I am confident, to an extent, that there will be enough land area. Sterness is just below Aberdeen in latitude and Seiskin/Vittoria are about the same latitude as Belfast. I certainly believe that the southeastern island will primarily consist of farming, and about half of the main island, up to Hopetoun (equivalent in lat. to Ayr), before slowly transitioning to pastoral farming further north. Historically, this would not be an issue, but today, yes it would be. The UK imports almost half of its food and I imagine Salide will have to import some as well. Probably from Ireland...
If I wanted Salide to be a perfectly planned, self-sufficient country, then I would have kept the population at 8 million when I originally started it. But the real world doesn't work out like that and I just have to deal with some of the consequences.
One final point on population. Ireland's population is almost 5 million. It peaked at about 8 million during the famine. If the famine had not happened, it is certainly possible that Ireland could have a population reaching 15 million or even 20. Concerning Scotland, terrain is definitely a factor. And the bloody English...
All of that being said... I will consider reducing the population a few million, maybe to 19. We'll see.
I chose Clifden (which connects to Galway) because I've been to Galway and because Galway is connected to the M6, that leads me to believe it would be a more important city than Sligo. However, given that Salide will definitely have an influence on Ireland's history, I could move the Thorpe-Clifden route to terminate in Sligo. Historically, there would likely have been a significant trading route to Sligo.
In 2018, I took a ferry from Belfast to Cairnryan (they don't dock in Stranraer anymore). By the time I got off the ferry, took a shuttle bus to Stranraer, hopped on the train, and switched trains in Ayr, it would have probably been quicker to just take the ferry straight to Ardrossan or Ayr
Sterness' metro has about 500k people at least and rail transport could get you from Buchanan to Sterness in less than an hour, so I don't think there's any need to have another ferry route from Buchanan to Scotland.