He is in Cork for the second annual UK-Ireland Summit, at which the focus is on greater co-operation across areas such as infrastructure, the cost of living and energy.
€5m worth of funding was announced on Thursday for 12 joint Ireland-UK cultural projects between now and 2030.
Mr Starmer said Ireland holds a “very special place” in his heart, and he counted the visit as both personal and political.
His time working in Northern Ireland meant he “fell in love” with the people and the place, and he said he has a deep “grá” for Ireland.
He said the summit has marked a “new era” in British-Irish politics and “turning the page on the turbulent Brexit years”.
Mr Starmer said this is important, as there is “more conflict now than I think any of us can remember globally”.
He said the new ambition will be to take the relationship “even further”, including a focus on maritime security and the economy.
Engagements throughout the summit will be crucial in an age of “dramatic uncertainty”, the prime minister added.
“The last two weeks have been a reminder of that uncertainty,” he said.
The summit will help to “tackle” the issues facing both countries, including “the cost of living and the concern that global affairs and global conflict will have an impact in the homes and households of every single person”, Mr Starmer said.
Speaking at Cork City Hall this evening, Taoiseach Micheál Martin said the relationship between the countries matters “greatly to all of us here this evening, and those we represent”.
The relationship between Ireland and the UK is shaped by geography, history, family ties, trade and a shared responsibility for peace
“As we gather, we are conscious also that this is a changing, more uncertain, and more unsettled world,” he added.
“In that context, the case for an active, engaged and close partnership between our two countries has never been greater. Relationships matter.”
Mr Martin said generations of Irish people have emigrated to the UK, with the personal and economic ties remaining strong. He said it was “very fitting” that the late Queen Elizabeth visited Cork in 2011 and the countries share key economic and business links.
“The relationship between Ireland and the UK is shaped by geography, history, family ties, trade and a shared responsibility for peace and prosperity in Northern Ireland,” he added.
He said both governments have a shared plan, which was agreed last year in Liverpool, across areas such as infrastructure delivery, housing, energy and climate. They announced €5m in funding for 12 cultural projects, including national archives research, exhibitions and an initiative to support people with disabilities to access cultural venues.
Protesters gathered outside Cork City Hall before the reception, with some holding Iranian, Palestinian or Irish flags to protest against the increasingly active role the UK is playing in the war in the Middle East, its position on Gaza and the delay in the Irish Government enacting the Occupied Territories Bill.
Speaking in Belfast earlier Mr Starmer said he had tried to “reset” relations between Britain and Ireland and it is in a “better place” now.
He also said the appointment of Peter Mandelson, who was a friend of convicted sex offender Jeffrey Epstein, to the role of ambassador to the US was his mistake.
“It’s me that makes the apology to the victims of Epstein,” he said.