**Taking the car to Britain and returning it at Gatwick require separate approvals.** A company may permit the ferry crossing while requiring the car to be returned to Ireland.

Enterprise is one company to contact directly: its current Irish terms allow mainland UK travel with advance authorisation and a surcharge. However, those terms don’t confirm an Ireland-to-Gatwick return. Ask specifically for an **international one-way rental**, with ferry permission, insurance cover and the total drop-off fee confirmed in writing. [Enterprise’s rental policies](https://www.enterprise.ie/en/car-hire-locations/ie/dublin-airport-i118.html).

My practical suggestion is to arrange two rentals:

1. Hire a car for your Irish touring and return it in Ireland.
2. Cross to Wales as a foot passenger—the Dublin–Holyhead route accommodates this. [Irish Ferries’ Holyhead information](https://help.irishferries.com/en/support/solutions/articles/80000971691-holyhead-port-directions-parking-public-transport).
3. Collect a second car in Wales, booking a one-way return at Gatwick. Confirm the collection location and opening hours before buying ferry tickets.

You mention September–October **2017**. I could verify current policies only, so I can’t name a company confirmed to have accepted that exact rental in 2017.