England, Ireland (Republic), and Scotland are close but separated by the Irish Sea. No direct land link exists between Great Britain and Ireland.
Direct flights connect London, Dublin, Edinburgh, Glasgow, Belfast. Airlines: Aer Lingus, British Airways, Ryanair, easyJet, Loganair. Book early for best fares.
No direct trains cross the Irish Sea. Take a train to a UK port (e.g., London to Holyhead), ferry to Dublin/Belfast, then train onward. London–Edinburgh/Glasgow: 4.5–5 hrs. Dublin–Belfast: ~2 hrs (Enterprise).
National Express, Megabus, Eurolines run intercity coaches. Longer but budget-friendly. London–Dublin–Edinburgh routes available.
Best choice depends on budget, time, luggage, and whether you need a car.