DERRY, Northern Ireland — As many across mainland Britain expressed guarded relief at the breakthrough in Brexit negotiations on Thursday, unionists in Northern Ireland felt a deep sense of betrayal after Prime Minister Boris Johnson signed a deal with the European Union despite significant objections from the lawmakers who represent them.

The new deal, which still needs the approval of the British Parliament, ignores red lines set out by the influential Democratic Unionist Party, or D.U.P., rejecting a separate Brexit arrangement for Northern Ireland from the rest of the United Kingdom as it leaves the European Union.

Broadly speaking, unionists and loyalists in Northern Ireland are in favor of remaining a part of the United Kingdom, while nationalists and republicans seek a united Ireland.

Under the draft agreement, Northern Ireland would remain aligned with the European Union’s single market to avoid a hard border on the island of Ireland. (Northern Ireland is part of the United Kingdom, while the Republic of Ireland is a member of the European Union.) But customs checks would be put in place at ports and airports for items coming into Northern Ireland from the United Kingdom.