He will fill the vacancy created by the resignation of Neasa Hourigan.
Deputy Ó Cathasaigh admitted the government has struggled with communications across the three parties since taking office.
He says he will work to improve the lines of communication and bring about joined up thinking and coherent action.
As is outlined in the programme for government, Deputy Ó Cathasaigh will also take the position of assistant government whip. The vacancy had arisen following the resignation of Neasa Hourigan back in June.
Ms Hourigan resigned from the role after voting against the government’s Residential Tenancies and Valuation Bill.
She said at the time "I hold significant concerns as to the impact of the government legislation on people living in precarious tenancies”.
Clear communication
In the past, Mr Ó Cathasaigh was a member of Waterford City and County Council for the Tramore local electoral area from 2019 to 2020.
Laura Swift was co-opted to Ó Cathasaigh's seat on Waterford City and County Council following his election to the Dáil.
Commenting on his appointment today, he stressed the importance of the role and the need for clear communication:
“It is no secret that this government has had a difficult start to its term. Some of that is from outside pressures, but we have to acknowledge as well that our communications across the three parties of government have not been good enough.
“I believe I can play my part in making those structures work better and to improve those lines of communication.
"We need joined-up thinking and coherent action between parties and departments to most effectively steer our way out of the current health and economic crises."
He said he hopes as a government a "constructive approach" will be taken to how Dáil business is conducted to make sure all voices are heard.
