The prime minister has said the centenary of Northern Ireland next year should be celebrated in a "sensitive" way.
During a visit to NI on Thursday, Boris Johnson addressed criticism of plans to celebrate the centenary.
It was the PM's first trip to NI since power sharing was restored in January.
He said while for many people it would be "something to celebrate", clearly there was the need to be "sensitive to the point of view of all communities".
Northern Ireland was created in May 1921 following the partition of Ireland.
Mr Johnson outlined the first stage of plans to mark the centenary - the establishment of a centenary forum and a historical advisory panel.
The prime minister said commemorations would be done "with the maximum possible academic focus and really try and get to the heart of the stories of all the possible perspectives and just try and build an appreciation of the past that has made us all."
'Nothing to celebrate'
During his visit Mr Johnson met First Minister Arlene Foster and Deputy First Minister Michelle O'Neill at Hillsborough Castle.
"I think it is an event for the whole of the Northern Ireland, looking forward to the future, looking forward to our young people having a place in the world, and that is what I want to see happening for our centenary plans," said Mrs Foster.
Northern Ireland Secretary of State Brandon Lewis said the centenary year would promote the region on a world stage.
Sinn Féin's Ms O'Neill said she warned Mr Johnson there was "nothing to celebrate" with next year's centenary.
She said it should be marked with a "reflective and honest" conversation about partition and its "failures".
The PM also met new Taoiseach (Irish PM) Micheál Martin at Hillsborough Castle.
It was his first face-to-face meeting with Mr Martin since a new government was formed in Dublin.
Mr Johnson said he had the "honour of meeting the taoiseach several years ago" and he was "very pleased to develop [their] friendship and relationship now".
Belfast trip a test of PM's optimism
By Stephen Walker, BBC News NI political correspondent
Boris Johnson presents himself as an optimist - a glass half-full prime minister rather than a glass half-empty one.
However, the bonhomie and jolly good nature he exudes can't hide the fundamental difficulties he faces when it comes to relations with the Stormont parties and Taoiseach Micheál Martin.
On Brexit, many around the NI Executive table worry that Northern Ireland will lose out and have big concerns over trading arrangements once the transition period ends.
There is much scepticism over what a "Boris Brexit" looks like - even his once-close allies in the DUP have real concerns about economic life for some businesses after the EU withdrawal.
On the fight against Covid-19 there are still worries that an influx of travellers from Britain could lead to a spike in the pandemic in Northern Ireland and the Republic - the new taoiseach sees things differently from his counterpart in London.
Mr Johnson will also highlight the additional funding for the Stormont executive, his support for a city growth deal and the five million pieces of PPE (personal protective equipment) that have come to NI from other parts of the UK.
As he tours the UK, he is a salesman for the union and not surprisingly he has also used this trip to flag up Northern Ireland's centenary in 2021.
Arlene Foster says events to mark the centenary can be inclusive. Michelle O'Neill says there is nothing to celebrate.
Welcome to Belfast, prime minister.
The taoiseach said he and Mr Johnson agreed to create new structures to ensure co-operation between the Republic and the UK after Brexit.
The transition period is due to end on 1 January and a new trade system will begin operating between Northern Ireland and the rest of the UK.
Last week, Cabinet Office Minister Michael Gove set out details of £355m in funding to ease trade going from Great Britain to Northern Ireland.
Mr Johnson reiterated his promise that businesses in the region would enjoy unfettered access to markets in England, Scotland and Wales.
The prime minister said there would be a trade border down the Irish Sea "over my dead body" following Brexit.
He said it was important to have the Northern Ireland protocol, which ensures the region follows some of the EU's rules to allow freedom of movement of goods and services across the Irish border.
Mr Johnson said he looked forward to developing the relationship between the UK and Republic of Ireland "in all sorts of ways - east-west, north-south, you name it".
'Cornerstone of progress'
Mr Martin said the pair would enjoy a warm working relationship.
"It is important for us both in terms of the British-Irish relationship which has been the cornerstone of much progress on the island of Ireland and between our two countries for well over two to three decades, and we want to maintain that," he said.
"It is challenging times ahead with Covid, Brexit, all of that."
The Fianna Fáil leader said it was particularly fitting the engagement was taking place so soon after the death of former SDLP leader John Hume.
"We remember John at moments like this because he did so much to facilitate these kind of meetings and make them much more regular in the normal course of events," he said.