What happens if an unauthorized subtenant is discovered living in a unit before the sale closes?

Under the Residential Tenancies Act, 2006, a tenant generally needs the landlord's consent before subletting their unit, so discovering an unauthorized subtenant before closing suggests the original tenant has breached the terms of their tenancy. This does not, however, mean the occupant currently living there can simply be removed on discovery; the Act's protections can still extend to whoever is actually occupying the unit, and how the situation gets resolved depends on the specific facts.

Because this is a legal issue connected to the existing tenancy, it is properly something for the seller, as the landlord at the time, to address before closing wherever possible, rather than something a buyer should assume will resolve itself once ownership changes. If you discover this during your due diligence, raise it directly with the seller and your own lawyer promptly, since it affects both what tenancy situation you would actually be inheriting and how straightforward, or not, dealing with the unit is likely to be after you take over as landlord.