Almost certainly, yes - regardless of your nationality.
Travelling from Belfast (in Northern Ireland) to mainland UK (and then on to Cornwall) will involve either a flight or a boat crossing.
ANY flights within or from the UK require ID, usually in the form of your passport, even if you are a UK national - and if you are NOT a UK national, then a passport WILL be absolutely necessary.
Even if you travel by ferry (boat) rather than a plane, travellers are still carefully scrutinised, and ID / passports routinely checked.
These procedures / checks may carry a little more weight than you would normally expect, as the routes from Northern Ireland (and Southern Ireland / Eire) are frequently attempted by people trying to enter the UK illegally: there are also (recent) historical issues to consider - although the 'troubles' in Northern Ireland (with terrorism / the IRA) are officially now ended, exit from Ireland still frequently comes under scrutiny when mainland UK is the destination, as IRA terrorists and sympathisers would often use these routes to gain access to mainland Britain.
In short, if you are travelling legally, with a passport, you should encounter no problems whatsoever: travel without a passport is at best extremly difficult and problematic, and most likely impossible.
Remember also that under the present worldwide anti-terrorism climate, anyone attempting to travel across international borders in a way that is suspicious in any degree are likely to find themselves talking to large officials in small rooms!
does my passport need to be in date to go to belfast
No
No, Belfast is in Northern Ireland which is part of the UK - you don't need a passport to travel from one part of the country to another.
No
No but you do need I.D.
no, a passport is only required for crossing of International Borders.
Yes
No but you will need photographic identity. Check wit the ferry company.
Yes - you will need to show it on arrival in Prague and to leave Northern Ireland.
You will certainly need a passport - since Ireland is off the UK mainland.
A passport money for ticktes and a plane would help.
This would depend on where you are flying from and how. Technically you do not need a passport within the UK but you will need photographic identity. Check with your flight company.