Originally Posted by fantasyfan
The United Kingdom refers to Scotland, Northern Ireland, Wales, Emgland and some offshore islands. Great Britain refers to the island of Britain and includes England, Scotland, and Wales. England is the political entity of the island of Britain excluding Scotland and Wales. This may change if Scotland declares complete national independence from the Crown--as it may do in the future--though that is by no means a certainty.
The Republic of Ireland comprises the 26 counties of the four provinces excluding the 6 counties in Ulster which are part of the United Kingdom. Particularly since the recent visit of the Queen to Ireland relations between the two countries have been exceptionally friendly and the Government of the Republic recognizes the entity of Northern Ireland and also allows any of its population to use an Irish passport if they so desire. Citizens of the Republic do not need a passport to travel to Great Britain and vice-versa.
If I may, I'll add that the province of Ulster has nine counties, three of which are part of the republic. At partition, only the counties of Ulster with a majority Protestant population remained in the UK.