Speaking at a Brexit seminar, Mr Callinan warned the Prime Minister must navigate “a delicate home situation”.
He suggested Britain could move away from a hard Brexit as it enters the crucial negotiations.
I see signs in the contacts that we’re having, both at EU level and with the UK, of a gradual realisation that Brexit in many ways is an act of great self-harm, and that the focus now is on minimising that self-harm
Mr Callinan said: “I see signs in the contacts that we’re having, both at EU level and with the UK, of a gradual realisation that Brexit in many ways is an act of great self-harm, and that the focus now is on minimising that self-harm.”
He added the British negotiating position is unclear with the Government at odds over how to approach the historic talks, with “no single, settled position” on Brexit in London.
Mr Callinan, who is heading the Dublin team co-ordinating the Irish government’s response to Brexit, said: “Even within the British government, there are very different views.”
According to the Irish Times, Mr Callinan said “oversimplistic views of what Brexit is and what it will do to Britain” are “front and centre” of public debate.
Mr Callinan leads the Brexit division with the Department of the Taoiseach to support Enda Kenny in his work on EU, Northern Ireland, British-Irish and International affairs.
It comes as the EU begins to set out its guidelines for talks with leaders calling for London to cut red tape for EU expats.
Advisers to the 27 other EU government leaders broadly endorsed draft guidelines for negotiations with Britain on its withdrawal.
In a mark of fireworks to come when talks get under way in early June, some pressed for clearer, tougher wording on how much Britain must pay to cover its commitments to the Union.
Under draft negotiating guidelines prepared by European Council President Donald Tusk after Mrs May triggered a two-year countdown to Brexit on March 29, the EU wants Britain to guarantee full rights to all resident EU citizens before it leaves the bloc in 2019.
The Prime Minister has called for a quick agreement to guarantee expats’ rights but the EU rejected that, saying legal certainty requires detailed work which must be wrapped into an overall deal. Each side has accused the other of using people as bargaining chips.