This is without a doubt my favorite sourcebook, and certainly one of the best ever-written by the folks at FASA. It's somewhat bittersweet however, since some of the best plotlines hinted at in the sourcebook were never developed in the Shadowrun mythology.
As one of the first sourcebooks produced for 2nd Edition, Tir Tairngire puts forward a lot of ideas that were really pretty ground-breaking for the time (and still hold up well)... written in BBS format, the history and current events files are interspersed with a LOT of discussion and argument by the different posters, which made sense. This was, after all, the hottest data posted on Shadowland thus far and everyone had something to say. I always thought it was ingenious to have post-wars between scientists arguing about how elves could possibly be immortal, where such a gift would lie in the DNA chain and what such a revelation would mean for the elves themselves.
There was also a lot of mystery introduced... considering that the Tir covers most of present-day Oregon, there was a lot of speculation in the book as to how the nation could suddenly have a culture that simply oozed with age and sophistication, when it was less than 50 years old? The postings about Crater Lake were fascinating as again, it was hinted that there was a connection between the land and the elves that pre-date the Awakening. Sperethiel, the elven language was treated the same way. These sort of threads are debated in a fresh and interesting way.
However, the greatest disappointment is that after the suppliment printed, next to NOTHING was done with any of the fantastic ideas present. Ehran the Scribe would make appearances, along with Harlequin and of course Big-D but next to nothing else was explored. No adventures were written with the Tir in mind for something like 8 years (which was released in the Corporate Punishment adventure book) and all the promising links between Shadowrun and Earthdawn (which was practically thrown in the reader's face) would lie dormant for even longer.
Of course this sad fact does nothing to diminish the excellent quality of the writing, and even moreso the 'feel' of the suppliment, which is all about those mysterious elves, only thrown in a post-industrial 21st century setting, a feat that has never been matched.