Two Nations on the British Isles: UK and Ireland, Separated by the Irish Sea

The British Isles — a geographical term for the archipelago off northwest Europe — contain two sovereign states:

1. United Kingdom of Great Britain and Northern Ireland (UK) — England, Scotland, Wales, Northern Ireland
2. Republic of Ireland (Ireland / Éire) — 26 counties, independent since 1922 (Irish Free State) / 1937 (constitution) / 1949 (republic)

The Sea Between Them: Irish Sea

• Extent: ~300 km (190 mi) long, ~150 km (90 mi) wide at broadest
• Area: ~47,000 km² (18,000 sq mi)
• Depth: Average ~100 m (330 ft); max ~315 m (1,030 ft) near Mull of Galloway
• Connections: North Channel (to Atlantic, 21 km / 13 mi at narrowest), St. George's Channel (south, to Celtic Sea)
• Major ports: Liverpool, Belfast, Dublin, Holyhead, Dún Laoghaire, Heysham, Warrenpoint
• Crossings: Ferries (Stena, P&O, Irish Ferries) — 2.5–3.5 hrs; Flights — 45 min

Islands Within the Irish Sea

• Isle of Man — Crown Dependency (self-governing, UK defense/foreign affairs)
• Anglesey (Ynys Môn) — Wales (bridged)
• Holy Island (Lindisfarne) — England (tidal causeway)
• Various small islands — Copeland, Lambay, Ireland's Eye, etc.

Not the British Isles: Channel Islands

Jersey, Guernsey, Alderney, Sark, Herm — Crown Dependencies in Gulf of St. Malo, geographically part of the European continental shelf, not the British Isles archipelago proper.

The Irish Sea is not a barrier but a corridor—millennia of migration, trade, raid, and cultural exchange flow across it.