Depends on the route. On the dual carriageways (similar to our interstates), you can maintain speeds, and the Google Maps estimates are probably fairly accurate. Unfortunately, dual carriageways are not the norm, with the glaring exception of Dublin to Belfast. When the economy was booming and the EU money flowed more freely, Ireland made attempts to upgrade its roadways. As a result, you'll find improved highways sporadically placed all over Ireland.

On the other hand, once you're off the dual carriageways, there are two factors that I believe you'll find will limit your speed. First, as others have already noted, you'll find yourself behind farm tractors (or other slow-moving vehicles) much more than you'd like.

Second, you will find that it's very difficult to maintain the posted speed on some of the unimproved N-designated roads, which are roughly equivalent in the pecking order (although not in construction) to US-designated highways in this country, and almost all of the secondary roads. These roads can be twisting and narrow, without shoulders and hard up against stone walls marking the boundaries between the roads and the farms. On some of these roads, the posted speed limit is the maximum speed any car driven by an American who is unfamiliar with the lay of the land should ever go, even during the day time and even assuming the road ahead is clear. Of course, at night, you should even go slower or risk that an unexpected sudden curve in the road might cause you to brush or hit one of those walls.

I think a good rule of thumb is to assume that you'll do no more than 40 mph going from place to place. You'll do better on the dual carriageways for sure, but once you've gotten 50 miles or so outside of Dublin, you'll find fewer of those.

And this is just as good a place as any to throw in the reminder that Irish and UK laws regarding DUIs are even stricter than in the US. If you're headed to a pub, walk.