LONDON -- The Irish ambassador to Britain warned on Tuesday that an unsatisfactory Brexit represented a danger to peace in Northern Ireland (NI).

"We should not underestimate the degree of worry and concern there is about the potential impact of Brexit," Ambassador Adrian O'Neill told an audience of policymakers and journalists at the Institute for Government (IFG), a social policy think-tank.

"If we left it to future negotiations and just allowed the situation in NI to drift... I think that would have been very irresponsible."

NI is part of Britain and has its border to the south with Republic of Ireland is the only land border with any other European Union (EU) nation.

NI suffered from a 30-year-long low-intensity terrorist war known as The Troubles. An agreement was reached between dissident republican groups and the governments of Ireland and of Britain in 1998. It is called the Good Friday Agreement (GFA) which brought peace to the region with a degree of power-sharing between parties.

The 20 years since then has been a period of stability for NI, and both the NI economy and the Irish economy have benefited from restriction-free cross-border trade.

The prospect of Brexit raises fears of a return to a hard border between NI and Ireland, with tariffs and customs checks as well as a legal, police and possibly military presence.

The implications of this could be to damage economic growth and to reignite republican sentiments in NI which might lead to a return to the era of The Troubles.

O'Neill said that he "took seriously" the assessment of the Policy Service of Northern Ireland (PSNI) and of the Garda Siochana (Police Service of Ireland) "and indeed the UK government's own assessment that the threat of dissident republicans remains severe."

The ambassador said: "Happily in recent years the peace cooperation between the PSNI ad the GI has been such that dissident republicans have had very little space to operate and while they did have a murderous intent and if they had opportunities to kill PSNI officers they would have availed themselves of the opportunities."

"They still have that intent but their capability has been much reduced and they do not have any significant traction within the broader community -- they do not really have a course," he added.