I've used insurance provided by premium credit cards in the past, however on more recent trips I've just opted for the CDW offered by the rental agency. I suppose I'm another of those in the category that Kent described as trading dollars for peace of mind while on vacation.
The credit card insurance seems to be provided by a "third party" as I had to call them for specific information, rather than the usual credit card customer service number. I've found that the provisions of coverage are somewhat confusing at the best of times, as there are exclusions for various countries and the card holder has to follow specific steps for the insurance to be valid (ie: decline the rental firm collision but accept the PL/PD). Also, the entire cost of the rental must be placed on that particular card.
I find it provides much more "peace of mind" to just pay for "hassle free" CDW and enjoy the trip. The insurance issues are one reason I prefer to use public transit most of the time, except for specific cases. Also, as I travel solo most of the time, a rental car isn't as cost effective as public transit.
Finally, the information posted by Steve regarding rental exclusions for Ireland and Northern Ireland isn't surprising. Apparently this is one of the more risky places to drive in Europe, a fact which was confirmed on my last visit there. The Garda suffered a week or so of horrendous traffic problems and fatalities. At that point, I was glad I was on a tour and someone else was doing the driving!
Cheers!