I am proficient in driving a stick/manual. But driving on the left side of the road, sitting on the right side of the car, narrow roads, winding roads, very narrow roads, sheep in the roads, did I mention really narrow roads???????- merits the extra $$ for an automatic, in my humble opinion. I spent 2 1/2 weeks last fall in England/Cornwall, Cotswolds and drove there. Just did 2 1/2 weeks in Ireland last month and drove Dublin up to County Donegal, then counter clockwise to Westport, to Ennis, to Dingle, to Kinsale, back to Dublin. Both of those fall trips I had splurged for the automatic and it was worth every penny. Just one less thing to worry abut when navigating and driving. I have been fortunate enough to have traveled to Ireland, Scotland and England previously over the last twenty years and drove a manual with each of those trips. I did fine, but the stress reduction of having an automatic these last two rentals was worth it.
FYI-I rented through AVIS and I pre-paid $1,800!!!! That rate was competitive to the other car rental agencies at that time. However, I kept checking the AVIS site, just wanted to see if their advertised rates had changed. I saw that the same car, for the same dates, had dropped $500. I immediately called AVIS customer service to tell them I wanted to switch but didn't want to pay the $50 cancellation fee (this was about a month before we left). The AVIS rep said I wouldn't have to pay the cancellation fee as I was cancelling and immediately renting another car. They refunded me the original pre-paid amount. I saved $500 just by spending a few minutes once a week checking on the price.
Good luck and enjoy your trip. Manual or automatic, it will be fantastic.