Welcome to the forum. Hope some of the answers below help. Of course, some answers are only to be had directly from the car rental companies. It takes time and homework to get the most out of your car rental budget.

1. Our credit card company DOES NOT cover rental in case of accident/damage; so, do we need both CDW & SCDW or just SCDW to cover us the most in case of accident or damage? * Both.

2. What car hire companies do you know of that allow their rental cars to cross between Northern Ireland and Republic? * All the car companies allow cross border travel. Some charge an additional insurance fee, some don't. 

3. Is it more cost effective to begin our road trip from Belfast or Dublin when hiring a car & crossing between Northern & Republic? * Depends. Will you be dropping the car off at the end of the rental in the same city? 

4. Is there a car hire company that covers all 4, undercarriage, windshields, tires and mirrors? * Since car companies are constantly changing their rules the only way to know is to individually check on the companies you are interested in and read the fine print and/or email them for more info. 

5. If no car hire covers all 4, undercarriage, windshields, tires and mirror, what is the most costly item of the 4 that should most definitely be covered, undercarriage? * Since I'm not a car mechanic I will take a wild guess and say undercarriage. 

6. Do you know what car hire companies covers the one thing in #5 we need to make sure is covered? * See answer 4. 

7. Overall, do you have a suggestion as to what car hire company would best fit our needs as far as coverage as well as price? * Price depends on time of year, type of car, length of rental, automatic or manual, extras. Since I am not privy to that info my best advice is to shop around, compare total final prices with all the extras you need added in. 

8. Do you recommend a 3rd party insurance along with the CDW or SCDW to cover what they do not cover? * In Ireland all car rentals include this. 

9. If so, what 3rd party insurance company do you recommend that is reputable? * See 8. 

10. When driving Ireland, is it best to tour clockwise or counter clockwise for someone who has never driven on the left and the other person who has only driven on the left a few times but with the steering wheel on the left of the car, not right? * if you are renting a car in Ireland the steering wheel will be on the right (if sitting in the car). Personally, I don't think it matters which way you go. What is your concern with it?