- It undermined the Union between Great Britain and Northern Ireland. The agreement recognised the Republic of Ireland's interest in Northern Ireland, which unionists saw as a step towards a united Ireland.
- It gave the Republic of Ireland a say in the governance of Northern Ireland. The agreement established an Intergovernmental Council, which included representatives from both the UK and the Republic of Ireland, to discuss matters of mutual interest. Unionists felt that this gave the Republic of Ireland too much influence over Northern Ireland's affairs.
- It weakened the position of the unionist community in Northern Ireland. The agreement led to the release of paramilitary prisoners and the end of internment, which unionists saw as a concession to the Irish Republican Army (IRA). Unionists also felt that the agreement failed to address their concerns about the discrimination against Protestants in Northern Ireland.
These factors led unionists to oppose the Anglo-Irish Agreement and to view it as a betrayal of their interests.