Northern Ireland has a very complicated relationship with flags. During the period of self government under the Unionist party (1921-72) the Union flag (Union Jack) was accepted as the national flag by the unionists but rejected by nationalists who flew the Irish tricolour. The government chose to treat the tricolour as the flag of the IRA rather than that of a sovereign state and the police removed it from public display. Although the tricolour was not technically illegal it was treated as a potential breach of the peace and taken down unless doing so would actually incite disorder (such as at a Gaelic football match). Flags were and still are used to mark territory signalling which ethnic group is dominant in an area.

My book Censorship in the two Irelands deals with this. The best expert on flags in Northern Ireland is Dominic Bryant who has written extensively on it.

The most well known case similar to this is when the Germans decided the Swastika no longer represented them, and it is illegal to this day.