**Returning the Irish car at Gatwick is the difficult part.** Permission to take a rental on the ferry to Britain doesn’t necessarily include permission to leave it there. For example, Enterprise allows authorised travel from Ireland to mainland UK, but its published European policy requires return to the pickup country. [Enterprise policies](https://www.enterprise.ie/en/car-hire-locations/europe.html)

I’d arrange **two separate rentals**:

1. Hire a car for your Irish travels and return it in Ireland.
2. Cross to Wales as a foot passenger.
3. Collect another car in Holyhead, drive through Wales and southern England, and arrange a one-way return at Gatwick.

[Enterprise’s Holyhead branch](https://www.enterprise.co.uk/en/car-hire-locations/uk/holyhead-u358.html) advertises one-way rentals within Britain. Confirm the Gatwick return, total drop-off fee, and collection arrangements before booking the ferry.

You mentioned **September 2017**; the policies I could verify are current, so they don’t establish what companies offered for those historical dates. I haven’t found a verified company offering the exact Ireland-to-Gatwick rental you describe.