The most notable is the Himalayan mountain range.
The Ural Mountains
yes there are no border restrictions from the republic,
There is no geographical feature that forms the border between Portugal and Spain. The Strait of Gibraltar and Mediterranean Sea separates Iberian Spain and Morocco. The Pyrenees Mountains forms the border between Spain and France and Spain and Andorra.
republic or Ireland
It has famous mountain ranges in the Balkans: Stara Planina, Rhodopes, Pirin, Rila. Daniube makes the Northern border. Many artesian wells of mineral water,big forests
None, really. You just drive over the border.
The Republic of Ireland. It has a border with Northern Ireland, which is part of the UK.
There is only one that is Northern Ireland (UK)
The Republic of Ireland has a common border with the six counties of Northern Ireland which is part of the UK.
The Republic of Ireland shares a land border with Northern Ireland which is part of the United Kingdom.
If you mean is there border control between the Republic of Ireland and Northern Ireland then no there's not you can just drive across the border and not even know