Disclaimer: I've only read 5 chapters and a handful of episodes. But I am Scottish so know some Scottish words.
A broch used to apply specifically to a special type of round tower made by the pictish people in Scotland around 1500 years ago. It became a word that just meant a tower or small castle in Gaelic over time.
Tuarach would mean something like towards or the facing so I imagine broch Tuarach means the castle that faces this way or it is near an ancient pictish broch and it is a clever play upon words that would mean the tower that faces the tower. Which made me chuckle when I worked it through in my head.
Lallybroch is likely a localised dialect scottish/Gaelic hybrid word. If you've ever used the word "lollygagging" to mean going slowly or reluctantly, the lolly from that comes from lally. So I'd translate that to English as the reluctant or slow tower. That's just from the words though so I'm doing my best guessing.
Hopefully it's explained later in the stories more fully.
Edit: whilst I was typing this someone posted an official answer above so I was a wee bit out.