Residential Tenancies Act 1986, ss 13-13B,
Your tenancy agreement has to be in writing, and it needs to be signed by both you and the landlord. The landlord must give you a copy of the agreement with their signature on it before the tenancy starts. If you renew the tenancy or if there are any changes, those also have to be put in writing.
Residential Tenancies Act 1986, s 13C
If your tenancy agreement isn’t in writing, or if it’s written but not signed, you still have all the same minimum rights that tenants have under the Residential Tenancies Act. The rule that the agreement has to be in writing is there to give a tenant more protection – it doesn’t allow landlords to get around their minimum obligations under this Act by avoiding having a written agreement.
If you do only have a spoken agreement, the terms of your agreement will be the rights and responsibilities set down in the Act, plus whatever you’ve agreed to verbally with your landlord.
Note: More than one person can sign the tenancy agreement. If several of you sign it, this means that the landlord can decide to hold all of you responsible, or just one of you, when a problem comes up. (The legal term for this is that you and the other tenants who sign are each “jointly and severally liable”.)