Corporate letting in a RPZ zone

Not sure I agree. This is down to the interpretation of the legislation. I think this is something to run past a solicitor. You are still entering a tenancy for a residential property in a RPZ, and relying on an interpretation that a corporate tenant does not meet the definition of a tenant under the legislation.

It's still a letting of a residential property in an RPZ. Unless you're arguing that they are not letting it for residential use if they are letting it to a corporation, in which case they'd likely be violating planning laws. The RTA "applies to every dwelling, the subject of a tenancy" and has no specific exclusions for such "corporate" lettings that I'm aware of. It does specifically exclude lettings to housing authorities, which is why a council let would not be subject to RPZ restrictions.

You can see all of the RTA exclusions here, but I don't see any that would exclude a letting to a private company to house one of their employees, so unless there is one, the RTA would still apply to such a letting, including RPZ restrictions.

That said, OP, if you haven't raised the rent in "a number of years", you can increase it by a fair bit. You can use this calculator to see what increase you can apply based on the date the rent was last set.

The RTA doesn't apply to lets to the councils, true. I suspect the corporate rental thing is just flying under the radar because no one in the chain cares enough to file a complaint about it; the tenant isn't the one paying the bill and a multinational isn't going to waste their time quibbling over a couple hundred euro a month in rent for a key employee.