In retrospect it is clear I mistook Dun an Ruigh Ruadh - just up the road - for this. Serves me right for not having the OS map of the area or that would have been two sites for the price of one. Anyway, to get to Dun Lagaidh it appears the traveller needs to proceed to the end of the tarmac in Loggie and follow the path NW (ish). The broch commands the most narrow part of Loch Broom. Apparently.
Anna and Graham Richie (once again) supply some detail by way of my battered Oxford Archaeological Guide to Scotland; in summary in seems that excavations undertaken during the 1960's established the first phase of the site to be a large, univallate stone walled fort founded c700 BCE and then 'vitrified'. Make of that what you will. Later the eastern section of the fort became the site for a powerful, defensive roundhouse.... a broch to you or I. Or perhaps dun. Finally, sometime during the 12th century, it seems this 'broch' was upgraded to serve as the donjon (keep) of a castle, the remainder of the fort similarly upgraded to serve as the bailey. So there you are, some two millennia of human occupation. Seems 'recycling' to save the environment is actually rather old hat. We've been doing it for years.