The ‘Brighton Up Your Day with the Provisional IRA' garment features a picture of the Grand Hotel in Brighton after it was almost destroyed in a Provo bombing.
The 1984 attack during the Conservative Party conference, which was aimed at then Prime Minister Thatcher, killed five people and injured 31.
Since the former PM's death, sales of the tasteless T-shirts have increased dramatically. Costing just £12.50, they come in a variety of colours and styles.
Ulster Unionist Michael Copeland said he was horrified by the sickening garments.
Although disagreeing with many aspects of Thatcher policy, the East Belfast MLA believes she should be respected in death.
He said: “Death is not something to be celebrated; terrorism is not something to be celebrated.
“Those buying these grotesque T-shirts obviously have no first-hand experience of terrorist acts and the devastation they cause.
“If they did, they would realise there is no cause for celebration.”
IRA man Patrick Magee, who was convicted of the Brighton bombing, still stands by his actions which almost claimed Margaret Thatcher’s life.
The 61-year-old former bomber has said: “I hurt human beings, I was involved in the armed struggle for a long time, I stand over my role in that struggle.
“But I have come to realise that in order to be involved in a conflict, in a war, you have this reduced view of people who you call your enemy.
“You see only the uniform, the political label attached to them or the nationality attached to them.
“You don’t want to be someone my age looking back and knowing you’ve hurt people and you’ve killed people. But I’ve found myself in a situation of conflict, I played a role in that. I thought there were no other options to our communities.”