Sinn Féin leader Mary Lou McDonald has said at least one of the Taoiseach's nominees to the Seanad should have been from Northern Ireland.
Micheál Martin announced his 11 nominations to the upper house of the Irish parliament on Saturday.
Former social protection minister Regina Doherty and traveller rights activist Eileen Flynn were among the nominees, who included a record number of women.
However, no-one from Northern Ireland has been included, despite citizenship campaigner Emma DeSouza being tipped.
The previous Seanad included Independent Unionist Ian Marshall, who campaigned against Brexit.
Ms DeSouza said she was dealt "a heavy blow" when she found out she was not included.
She said: "I had hoped to be successful in obtaining a nomination to the Seanad as an independent that represents the Good Friday Agreement; instead, no-one from the North was appointed."
She added: "Taoiseach Micheál Martin did confirm on Saturday that there will be a newly formed unit within the department of an Taoiseach to work towards a consensus on a shared island, but this announcement was followed by the row-back on a Northern nomination, immediately undercutting the concept of a shared island and reigniting fears that we in the North will be left behind."
Fine Gael councillor Emer Currie is one of the nominees. Her father, Austin Currie, was a founding member of the SDLP.
He joined Fine Gael in 1989 and served as a TD in Dublin West in the Republic of Ireland for 13 years.
Ms McDonald said at least one person from Northern Ireland should have been selected.
"When Ian Marshall came into the Seanad, we supported his nomination. I think people were taken aback by that but I think it is important that northern society is represented within the Oireachtas," she told BBC NI Sunday Politics.
"I also think it is important that people from a Unionist tradition are included. So that is a disappointment."
Ms McDonald said Sinn Féin has two Senators from Northern Ireland in the current Seanad.
"It is very important that national politics is reflected in the Oireachtas so the new Government have a unit in the Taoiseach's department about a shared island ... Well, they're not off to a very good start in that regards.
"In my view they should have made at least one appointment from the North."