Margaret Thatcher was a hugely divisive figure in British politics. And for the people of Ireland, and especially the north, the Thatcher years were among some of the worst of the conflict. Her policy decisions entrenched sectarian divisions, handed draconian military powers over to the securocrats, and subverted basic human rights.

Thatcher refused to recognise the right of citizens to vote for representatives of their choice. She famously changed the law after Bobby Sands was elected in Fermanagh and South Tyrone. And when I and several other Sinn Féin leaders were elected to the Assembly in 1982 we were barred from entry to Britain.

Margaret Thatcher's government defended structured political and religious discrimination and political vetting in the north, legislated for political censorship and institutionalised, to a greater extent than ever before, collusion between British state forces and unionist death squads.

It was under her leadership that in 1982 that the Force Research Unit (FRU) was established within the British Army Intelligence Corps. This unit recruited agents who were then used to kill citizens. Among them was loyalist Brian Nelson, a former British soldier and member of the Ulster Defence Association.

Nelson travelled to South Africa where he helped negotiate a deal that saw the UDA, UVF and Ulster Resistance acquire AK-47 automatic rifles, pistols, grenades, and RPG rocket launchers in late 1987 or early 1988. The Thatcher government was across all the details of this shipment. Its impact on the streets of the north is evident in the statistics of death. In the three years prior to receiving these weapons the loyalist death squads killed 34 people. In the three years after the shipment they killed 224 and wounded scores more.

But it was the killing of human rights lawyer Pat Finucane in February 1989 that reveals the depth of the Thatcher government's state collusion policy. At every level of his killing, British agents and agencies had a hand: the leader of the UDA group that carried out the killing was a Special Branch agent, as was the man who confessed to being the gunman, and the man who supplied the gun. And, of course, Nelson provided the intelligence.

Thatcher will be especially remembered for her shameful role during the epic hunger strikes of 1980 and 81. The Thatcher government believed that the criminalisation of the republican prisoners would break the republican struggle. It was not interested in a resolution.

The events of that awful summer of '81 polarised Irish society, north and south. It was a watershed moment in Irish politics. Government policy during the 1980s was little more than a war policy, aimed at defeating or isolating republicanism. Its strategies included the shallow and ineffectual 1985 Anglo-Irish Agreement which was about creating a political alliance involving the Dublin establishment, the SDLP and the British to defeat Irish republicanism.

Shoot-to-kill actions by British forces also significantly increased. This was most evident in the shooting dead of three unarmed IRA activists in Gibraltar in March 1988. It is my view that Thatcher authorised the killings at Gibraltar. Later, when the BBC and ITV scheduled two programmes about Gibraltar, Thatcher tried to stop them. She was "outraged" when the programmes went ahead. Later that year she introduced the broadcasting ban on Sinn Féin, which prevented viewers and listeners hearing my voice.

In 1990 Thatcher authorised the then British secretary of state Peter Brooke to reopen back-channel negotiations with republicans. We were wary of this. However, for almost a decade Sinn Féin had been patently trying to build a peace process and unfolding events on the world stage – including the dismantling of the Berlin Wall and the reunification of Germany, and the release of Mandela – were evidence that governments, and apparently intractable situations, could change. So we agreed to reactivate the back channel.

For Thatcher it all ended months later in November 1990 when she was forced to resign. She was evicted from Downing Street with all the ruthlessness, treachery and warped humanity of what passes for high politics. Thatcher's 11 years of dictating British policy in Ireland was a legacy of bitterness and entrenched division. Her Irish policy failed miserably.

• A longer version of this article appears on Gerry Adams' blog