Taoiseach Micheál Martin believes the relationship between the SDLP and his own party, Fianna Fáil, will continue to “grow and strengthen”.
In February 2019, the SDLP voted overwhelmingly in favour of a partnership with Fianna Fáil.
However, the Fianna Fáil leader said this will not affect his role as a co-guarantor of the Good Friday Agreement.
Mr Martin also dismissed claims he was ignoring unionism by not electing a unionist to the Seanad (Irish Senate).
He also believes a border poll “out of Brexit” will be “too divisive”.
Speaking on BBC NI’s Sunday Politics programme, Mr Martin said he will play his role as taoiseach “down the middle”.
“I will continue honest and objective leadership in respect of the north,” he said.
“I will be proactive in a positive and constructive way with Northern Ireland.”
Mr Martin said that the SDLP has experienced “a renaissance” in Northern Ireland and said that Fianna Fáil will continue to “give support to the SDLP”.
The draft deal that was agreed to establish a government in the Republic of Ireland sought the establishment of a new shared-island unit within the Department of the Taoiseach.
The paper for government said this unit will be set up to work “towards a consensus on a shared island”.
Mr Martin said that there is a commitment to “build relationships” with the Stormont Executive and the UK government to achieve “greater connectivity” on the island of Ireland.
Mr Martin had been previously accused of “letting down the unionist community” in Northern Ireland by not including a unionist in the Seanad.
‘I am not talking to myself’
Ian Marshall, who in 2018 became the first unionist elected to the Seanad (Irish Senate), called Mr Martin’s commitment to a “shared island” a “farce”.
“How can you have a shared island if you only talk to yourself?”.
Responding to Mr Marshall’s comments, Mr Martin said it was “a disproportionate comment” and said “having a (unionist) senator is not the be-all-and-end-all in terms of having a relationship”.
“There is no issue at all with Mr Marshall, but there were political circumstances prevailing on this, particularly in the parties that came together to form a government,” Mr Martin said.
“Engagement with unionism is far wider than that,” he added.
‘Border poll unnecessarily divisive’
However, Mr Martin said that “out of Brexit came a demand for a border poll”, but believes it “would currently be unnecessarily divisive”.
The Fianna Fail leader said “it’s easy to call for a border poll” but says the “nuts and bolts of how does one share an island” need to be understood.
Mr Martin said that the most effective way forward is to “build on relationships” and to “do research” into the practicalities of these issues.
“More substance around these issues is needed and a greater understanding of the implications of what some people are calling for,” he said.
You can see the full interview with the taoiseach on Sunday Politics on BBC One on Sunday at 10:15 BST.