Car Insurance and Driving Conditions

In the UK, insurance that covers people and things outside the car is called third-party insurance. It's a legal requirement and is included in the overall cost of the car hire. Insurance that covers the car itself will either be your insurance or insurance sold by Hertz.

You're not planning on taking the car to Belfast are you?

yes, planning to take the car to Belfast as well. Will there be any issue?

will Hertz allow you to take it to Northern Ireland?

Have you looked at the cost of the ferry?

Hertz seems okay with taking car on ferry to Belfast as it’s still winning UK. If I have to take it to Ireland, I would need an authorization.
https://www.hertz.com/rentacar/reservation/reviewmodifycancel/templates/rentalTerms.jsp?KEYWORD=DRIVINGRESTRICTIONS&EOAG=LHRT50

Ferry cost is very high, it comes upto 450 USD for two people on a round trip with car. Quite expensive.
One other option I could think of would be to have a car parked in a safe location before getting on to ferry.

Look into flying from Newcastle, Edinburgh or Glasgow to Belfast

Why not lose the car at say Newcastle and fly to Belfast and hire another?

If taking the ferry over to Northern Ireland (as opposed to flying) it is vastly cheaper to off rent at Glasgow, take the bus or rail/bus combo to Northern Ireland and rent another car in the province.

You can also fly from both Glasgow (Thursday to Monday on Loganair) and Edinburgh (Edinburgh started on 4 November on Easyjet.on a Monday and Friday) to the City of Derry airport.. Very handy for the Causeway coast.

Everyone on this forum focuses exclusively on the two Belfast airports.

Hannons Bus will take you from Glasgow to Belfast driving on and off the ferry with you, so totally painless. Your luggage stays in the hold of the bus for the ferry crossing.

hey hey
check if you will need an IDP (international driving permit) AAA.com/IDP in the country you are planning to drive in, get it ASAP
some countries require it even car rental kiosks. definitely if you are you are stopped by police, expensive fine. good luck & enjoy
aloha

You don't need an IDP for the UK.

All an IDP does is translate your drivers license into a standard format for those who don't speak English.

Where do you plan to visit in Northern Ireland besides Belfast? You may not need a car or only need it a few days.

Hertz may demand a letter from your credit card company confirming your coverage.