Unionists fear losing identity and land, and facing nationalist triumphalism and potential retribution in the event of a united Ireland, research suggests.
A new report based on contributions from unionist politicians, loyalists, community leaders, former soldiers and clergy has identified seven key areas of concern.
These include a loss of identity, triumphalism by nationalists, retribution on ex-security forces, land being taken from unionist farmers and a return to violence.
The remaining concerns identified were around rejoining the European Union as well as the implications for healthcare, welfare and the economy.
Fianna Fáil Senator Mark Daly has published the report, which is entitled 'Unionist Concerns and Fears of a United Ireland: The need to Protect the Peace Process and Build a Vision for a Shared Island and a United People'.
Senator Mark Daly sought out Unionist opinion on a United Ireland over 18 months.
Contributors include the Rev Kyle Paisley, son of the late DUP leader Lord Bannside, former Irish rugby international Trevor Ringland, ex-UUP leader Mike Nesbitt and the former Presbyterian moderator Norman Hamilton.
Meanwhile, James Wilson, a former soldier who served in Northern Ireland during the Troubles, conducted focus groups with the independent Orange Order, a loyalist flute band and Army veterans.
Senator Daly says it is the first research report by any member of the Dáil or Seanad on this subject.
He said the "mother of all fears" that he heard in the 18 months spent compiling the report was that unionists fear their home would effectively become a foreign state.
Senator Daly urged the heeding of the advice of former SDLP deputy leader Seamus Mallon that "nationalists need to show generosity if they're ever going to persuade unionists of the benefits of a united Ireland".
He has also urged the Irish Government to address these concerns by establishing a New Ireland Forum 2 to look at all the issues in relation to a united Ireland.
Under the 1998 Good Friday Agreement, a referendum on Irish unity can only be called by the Northern Ireland Secretary if there is evidence that a majority in the province would support a united Ireland.
Senator Daly said the implications of unification should be examined and known before a border poll is called.
"The Brexit referendum has taught us an important lesson," he said.
"You do not hold a referendum until every probable outcome has been examined and prepared for, where possible.
"The hard-won peace we all enjoy on this island is at stake.
"The holding of a referendum without proper preparation and engagement, particularly with the unionist community, would lead to the fulfilment of the warning 'policy neglect seldom goes unpunished'."
Additional reporting MIcheál Lehane