They aren’t in any way owned by Brussels. Ireland or rather the republic of Ireland gets a veto vote at the confederation that is the eu. Ireland has set the limits fir the eu negotiating team. In contrast northern Ireland does not have that ability in the English union
The British would be wise to find a way to finesse the issue. This could be done by establishing Northern Ireland as a special customs zone, permitting free trade and movement with the Irish Republic but re-establishing customs and immigration barriers between Northern Ireland and the rest of Britain.