Well, some months ago the news was
Sinn Féin will demand a referendum on the future of Northern Ireland if there is no deal on Brexit, because a hard border would be erected the instant the UK crashes out of the EU under World Trade Organization rules, the party’s leader has said. [...]
The principle of self-determination for the people of Northern Ireland is enshrined in the Good Friday peace agreement and allows for a border poll if there is evidence that a majority in Northern Ireland would support the reunification of the island.
Whether this is an empty threat or not, time will tell. The political-pie divide seems to be slightly against Sinn Fein at the moment:
Sinn Féin has seven MPs at Westminster, representing all border constituencies, West Belfast and mid Ulster, although it does not sit in parliament because of its abstentionist policy. It is the second biggest party with 30% of the vote, just behind the DUP at 36%.
I guess Sinn Féin is waiting to see what the actual outcome of Brexit is and possibly even how the border with Ireland will look in practice after that.
Also, if we go by demographic projections (of last year) 'Catholic majority [is] possible' in NI by 2021. The current Protestant majority is based on older citizens, so demographics are working against it:
More recent figures from 2016 show that among those of working age 44% are now Catholic and 40% Protestant.
The difference is even more marked among schoolchildren with 51% Catholic, 37% Protestant.
Only among the over 60s is there a majority of Protestants with 57%, compared to Catholics on 35%.
This demographic trend toward a Catholic majority is not however a guarantee of tipping the political balance for union with Ireland, even in the future.
Looking at the last census in 2011, Mr Nolan points out although 45% identified as being from a Catholic background, only 25% claimed an exclusively Irish identity.
Also news from late last year mentioned this poll:
A LucidTalk/Times newspaper poll on Friday showed that if there was a referendum next year, 30 percent of voters would definitely vote for a united Ireland if Britain leaves the EU on the terms Theresa May is currently proposing, but that would rise to 48 percent if Britain quit the bloc without a deal.
So Northern Ireland voters are sensitive to more than their mere religious affiliation when making such decisions.