Irish law textbooks are often incredibly expensive, and you usually won't get away with purchasing one for less than €100. By contrast, I suppose if you wanted to get a feel for law more generally you could pick up a much cheaper English textbook which often go for in and around €50.
Speaking about TCD more specifically, I would say that while the library is well-stacked it will tend to get pilfered come time for essays/exams and if you aren't amongst the people to nab them first you'll have to wait and put books on hold etc etc, which can be somewhat frustrating. If you can afford to buy them then I think it's always preferable to do so, but if you can't then I'd agree with ravetastic when they say that the library is probably generally well-stocked enough to get you by.
As for what you asked about freshman year reading, the majority of law ends up as reading cases. Textbooks are great for giving you an overview of an area of law, and oftentimes if your lecturer has written the textbook you're using you'll find a substantial intersect with the structure of the relevant course. However, most lecturers seem to prefer relying on the primary sources of law (caselaw/legislation), supplemented by some secondary sources (usually articles), as they teach their courses. Basically, prescribed textbook reading isn't necessarily the most important thing on a law course and it really doesn't occupy centre stage. They're an excellent resource for a student getting their head around the area for sure, but it all comes back to the case law in the end, whether you read about it in a textbook or not.