Short term tenancy is one which is fixed for a period of 120 days or less. This period is anticipated to become 90 days if proposed amendments to the Residential Tenancies Act are passed as law.
The landlord and tenant must agree in writing before commencing the short term tenancy that the term will not be extended or renewed beyond a total of 120 days. Once that agreement is in place the provisions in the Residential Tenancies Act about increasing rent do not apply.
If the landlord and tenant agree to extend or renew the tenancy so that it becomes greater than 120 days, then all those provisions of the Act will apply from day 121 onwards.
The Residential Tenancies Act does not apply where the premises are let for the tenant’s holiday purposes. If you want to let your home to someone wanting to use it for a lengthy sabbatical or holiday purpose, you could elect to ‘contract into’ the Residential Tenancies Act under section 8 to avoid any doubt about the terms of the letting arrangement.
Source Dept of Building & Housing