It's a VERY long story, so look it up, but basically, it was the British. They took Ireland's land.
Both. Belfast is on the island of Ireland, in the part of it that is known as Northern Ireland.
It is part of the island of Ireland. Great Britain is made up of England, Scotland, Wales and the small surrounding islands. Ireland is a separate landmass and is not part of Great Britain. However...
Someone with a driving license from the Republic of Ireland can drive in Northern Ireland.
Northern Ireland is part of the island of Ireland.