Sure, I would be glad to provide tips for your trip to Derry, Belfast, and Bangor in Northern Ireland:

Best Time to Visit:
The ideal time to visit Northern Ireland is during the summer months of June, July, and August. Spring (April & May) and Autumn (September & October) can also be lovely but with more unpredictable weather.

Route and Accessibility:
You could start your journey in Belfast, then Bangor and lastly visit Derry. All these cities are well connected by rail and road networks. Public transport in Northern Ireland is reliable and easily accessible which includes buses, trains, and taxis.

Altitude and Acclimatization:
These cities are at a low altitude, so acclimatization is not an issue.

Accommodation and Booking:
There's a wide range of accommodation available in all these cities to fit every budget. It's best to book your accommodations in advance, especially during peak travel seasons.

Clothing and Essentials:
Pack for changeable weather. Always have a light rain jacket, comfortable walking shoes, and umbrella handy. Sunscreen and sunglasses might also come in useful for sunny days.

Health and Safety:
Northern Ireland is generally safe, but it's always good to be aware of your surroundings and protect your personal belongings. It's also advisable to have travel insurance.

Local Etiquette and Respect:
Northern Irish people are known for their friendliness. Being polite and respectful goes far in these parts. Respect local customs and traditions, always ask permission before taking people's photographs. Try to avoid discussing sensitive political issues.

Visiting the Giant's Causeway, Titanic Belfast, Peace Wall and the stunning coastal routes are highly recommended. Enjoy your trip!