Yes, Taras are a matter of absolute pot luck. I did a "watch" on them for two months last year for a friend who is accumulating information on them for an eventual IRRS article, and there was barely a day they went according to anything remotely close to the timed paths. On at least one occasion i heard one at exactly the right time, and rushed to the window to see, only to discover that it was pure coincidence - in fact, what i heard was one going the opposite way, itself two hours out of path.
Regarding timber and containers in and out of Ballina and Westport, with many passenger trains to cross on single lines, I guess that these go very much more to timetable. Someone with a current WTT (not me) would be able to elaborate.
Kildare is a good place to spot these.