Just to be clear, a roundabout is what I've been taught to call a rotary, which I guess is short for rotary junction. It's when several roads meet and instead of putting up stop signs or traffic lights, the traffic is made to flow in a circle with exits to the roads at various points. In Canada, rotaries are infrequent and used only for major roads like highways (with the exception of Sherwood Park, Alberta). In Scotland, the are frequent, and in Ireland it seems to me they are even more frequent.
Let me put it this way. In Dunoon, there is a street called John Street, on which lies my bank and the supermarket. John Street forms a T-intersection with Alexandra Parade, a street that runs along the waterfront. In any Canadian town of Dunoon's size, this intersection would have a stop sign on John Street. Or perhaps given that John Street is busy enough, there would be a three-way stop at the intersection. This is not the case. Instead, there is a little concrete circle, only about 3-4 feet in diameter, in the centre of the intersection. It is a roundabout, albeit a tiny one. There is another roundabout farther down Alexandra Parade by the ferry where the street merges with Argyll Street (the main drag) to form a street called Victoria Parade. And according to my map of Dunoon, there's another roundabout farther down that street. So Dunoon has three roundabouts in a town of about 9,000 people.
Roundabouts are even more frequent out on the highways, known as motorways in this country, and to my amusement, dual carriageways when it's a divided highway. I picture horse-drawn carriages zipping along at 70 mph. Another similar out-of-date term is "telegraph pole" for any pole, be it telephone or electrical. Anyway, from what I've seen of the motorways in Scotland and Ireland, instead of using a ramp-exit system with over/under-passes like is used on Canadian highways, roundabouts are used whenever roads meet. There are sometimes over/under-passes, but they are still part of a roundabout. These large roundabouts don't have little concrete circles in the middle obviously, instead they have raised banks of grass with plants and trees.
On a bus from Dublin to Kilkenny, near the town of Naas, my bus went through three roundabouts in succession. I'm not joking - through one roundabout, then a few hundred yards of straight road before another, and then repeat. I have no idea how much the bus' direction of travel was altered from its original course after all that whirling around - we could have been going back again for all I knew.