We rented from SixT in 2015 and didn't take the full/high coverage. We had one of those 'scrapes' that we hadn't noticed (brush along the roadway)? IAC, SixT charged us €599 for the 'scrape'. Before SixT we'd always used Hertz, pickup/return at the airport. No issues with Hertz in over a couple dozen trips. (Why did I try SixT? -- don't ask. Never again.) But, yes, if you read up on rental car insurance you'll have found that Ireland is one of the "nasty 6" when it comes to rental car insurance. There's just no way (we've found) to beat the need to get their insurance.
When we are spending any days in Dublin at the start or end of an Ireland trip, we do NOT take the rental car for those days. We use the bus to get between Dublin & the airport.
On automatics. Yes, you'll pay more but if you're not used to a stick shift, it MAY be worth it given the narrow roads and the "other side" driving. We've finally worked our way to feeling ok with a stick shift but in the early trips always opted for automatic. It's really important to specify it (and check that you have it reserved). On one of our early trips we waited 4 hours for them to drive "THE automatic" up from Cork to Dublin. We waited. We watched as the rental desk offered the same "upgrade" to person after person to take a standard transmission. We were the only stubborn ones to wait it out. Not ideal to waste 4 precious travel hours sitting at the rental desk.