Eight and nine passenger People Movers (which are really best for six passenger with luggage) are typically Manual Transmission.  Automatics are particularly sparse in that size range.  Seven passenger (Ford Galaxy) automatics are more common but are hardly suitable for six -- particularly if they have luggage.

Do you use a Credit Card that covers you in Ireland for the insurance?

If that's so, go with whomever has the best price and terms.  Dooley places a smaller 'Hold' on your card than Hertz (3,00 Euro as opposed to 5,000), but Hertz MAY waive that if you are a Member of their 'Gold Plus' plan.  Membership is Free.

If you buy the Full Insurances, go with either of the following:

Dooley have an EXCELLENT staff in their New Jersey Office that is VERY highly regarded:

1 800 331 9301 or, their web site: https://www.dandooley.com/

Dooley has an Agent (Fox Car Hire) that Michele and I have used because they are sometimes cheaper than Dooley's regular prices:  

https://www.foxrentacar.com/en/locations/ireland/shannon-airport.html

Hertz: Try Conn's Ireland Car Rental. He's based in Kansas, of all places, but, he's been a Hertz agent for MANY years and lots of people I know ( Murphy and Bit Devine, to name two!)  rave about his prices and service.  Since he only offers FULL coverage, I've not used him. My dealings with Hertz have been direct with Hertz Ireland.

http://www.connsirelandcarrental.com/ Phone: (801) 649 5353 316 315 5993