some form of ID is recommended and if you are not a UK or Irish citizen photo ID would be a good idea
The ID that is needed to fly to Dublin from Prestwick on a Ryanair flight is a passport.
Great Britain and Ireland is a common travel area and no passport is needed to travel between the two, only photographic ID (this is only required for check in and boarding)
No but photo ID is a good idea.
Probably photographic identity such as a passport or suitable driving licence. Requirements depend on the ferry company.
Most airlines request photographic ID for travel now. An airline is unlikely to accept a buss pass ID. A passport is advisable.
None
Not officially, but you may be asked for ID by Irish immigration officials.
Strictly speaking, you won't need any ID to get on the ferry from Portsmouth to Guernsey. However, if you are not a British citizen, it might be best to take your passport with you.
not if you are an Irish or UK citizen. Other nationalities may be asked to produce passports/id cards
you need to check with your airline - most require passports
Check with the ferry operators