Rental income of the type that you describe (where you are also living in the property) will be assessed in full as income - as fairy1 said, there is no disregard.
Here's the relevant paragraph from the Means Assessment guidelines on
www.welfare.ie
"Any cash income from letting out a portion of a house that is not personally used or enjoyed is assessable against the claimant as income. For example, a room might be rented on a seasonal basis to students, ‘Air B n B’ customers, or for use as a business office."
The disregard of €120 pw applies to earnings from work, generally work of a rehabilitative nature.
Means assessment guidelines can be found here -
www.welfare.ie