The sacrifice for travel
Boswell's life in Edinburgh is pretty comfortable, so it is ironic that he and Johnson should subject themselves to the tedium and difficulty of traveling, especially since (as Johnson sometimes bemoans) they aren't really heading toward any specific objective. Instead, the travel itself is the object of their pursuit. They want to travel for the experience of traveling, and although they make the Hebrides their goal, it is the experience of passing through the highlands that Boswell celebrates most.
Johnson's ironic character
Johnson likes traveling for different reasons, leading to conflicts where they have to discover what it is that the other person enjoys. After some entries about unpleasant encounters between them, the reader learns what it is about Johnson that rubs Boswell the wrong way. He wants to have fun by doing things, whereas Boswell wants to escape things and "fun" altogether to be amazed by the silent beauty of nature.
Boswell and Lady Macdonald
Lady Macdonald is married to Sir Alexander, which is a pretty good reason for Boswell to keep his distance. But in fact, her marriage makes Boswell more attached to her emotionally, because he cannot help but to compare the life she leads with Alexander to his own life. He cannot help but feel a chemistry between them, and he wants her, perhaps detecting boredom in her demeanor. He suspects that if she could travel with him, she might love him, but the whole endeavor is ironic because he cannot have her.
The irony of home
The journal details a side trip where Boswell and Johnson visit the Boswell estate in Auchinleck. He doesn't stay there, so there is an irony about what home means to Boswell. He isn't at home in Auchinleck, and although he is more used to Edinburgh, the case could be made that he only feels at home when he is amazed. Perhaps his real home is the mountain range he desperately strives to see. He feels at home when he is not around anyone who knows him.
The irony of return
When they return to their lives, they are able to see the objective value of Edinburgh. Their lives there are imbedded in community where they serve social roles, and where they are bound by politeness and social constructs. They are back in the domain of society. Although this was completely normal and taken for granted at the beginning, they now see how radically different city life is. They see urbanization for its remarkable progress, and they contrast the beautiful wilderness to the order of the charming city.
