What is the difference between a co-tenancy agreement and a standard lease when buying a multi-tenant property?

A standard lease sets out the tenancy relationship between the landlord and the tenant or tenants named on it, and this is the document that determines who is legally a tenant with rights and obligations toward you as landlord under the Residential Tenancies Act, 2006. A co-tenancy or roommate agreement, by contrast, is typically a private arrangement among the co-occupants themselves, covering things like how rent gets split, who has which room, and shared household responsibilities, and it generally does not bind the landlord at all.

For a buyer taking over as landlord to a multi-tenant unit, what actually matters legally is who is named on the lease itself, since that determines your obligations and who can enforce tenancy rights against you. A private roommate agreement between the occupants, however detailed, does not change that underlying legal picture. Review the actual lease closely to understand exactly who your tenants are and what you owe them, rather than assuming a separate roommate agreement covers, or limits, your responsibilities as the new landlord.