The Union Jack, the “official” flag of Northern Ireland, is the symbol often displayed by unionists, whereas the Irish tricolour is the symbol often displayed by nationalists. Although the Irish tricolour was designed to symbolize peace between Catholics and Protestants and unity in allegiance to Ireland, it is clear that its presence amidst support for the Union Jack flag has contributed to intergroup tension rather than peace.
Many nationalists, for example, associate the Union flag with British domination and oppression. The Irish tricolour has been labeled by some unionists as a “rebel” flag, and there have been repeated attempts to ban its display. Each group has perceived the display of the opposing group's symbol as a provocation, and these symbols are immediate and explicit indicators of perceived loyalty or disloyalty, and thus, symbols of ingroup/outgroup membership.
David A. Butz, National Symbols as Agents of Psychological and Social Change, Political Psychology, Vol. 30, No. 5 (October 2009), pp. 779-804