As far as Ireland goes, there are very nice double-decker tour buses that you can get on and off as you please. In fact most big cities in Europe have them. What we would do is ride all the way round the city first thing in the morning, making notes as to what we wanted to see in detail, then use the all-day bus ticket as our personal transportation to and from each attraction. If it goes anywhere near your hotel, you can even drop off purchases, then get back on the next one. And I agree with Bluefire that you often hear interesting things that aren't in the book.
peace,
ES