Where is the status of "licensee" defined in Irish law?

I was inspired by this thread where there is some debate as to whether a lodger is a licensee or a tenant, and if this status can change if the owner moves out.

The Residential Tenancies Act doesn't really define the term except in relation to student accommodation. The Act (confusingly in my view) talks about licensees being reasonably allowed to become tenants without ever defining what a licensee is.

The RTB website defines a licensee as below but there are no sources given:

1. persons staying in hotels, guesthouses, hostels, etc.,
2. persons sharing a house/apartment with its owner e.g. under the ‘rent a room’ scheme or ‘in digs’,
3. persons occupying accommodation in which the owner is not resident under a formal license arrangement with the owner where the occupants are not entitled to its exclusive use and the owner has continuing access to the accommodation and/or can move around or change the occupants,
4. persons staying in rented accommodation at the invitation of the tenant.

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Is there any relevant legislation or case law?

NoRegretsCoyote said:

The RTB website defines a licensee as below but there are no sources given:

Click to expand...

That's a list of examples, not a definition.