IE to UK transit then my duration to stay in UK

[bearteacher.tw]

I arrived at Dublin airport then transit to Edinburgh one week ago.

I entered the passport check at Dublin and the officer gave me a specific date, 20 March 12, written on the entry stamp.

He asked me how long I plan to stay in UK and also saw my return ticket.
My return ticket was booked on the flight on 20 March and it departed from Dublin. I think this is the reason 20 March from.

But now I am in Edinburgh and would like to stay one week longer. I am quite confused about IE and UK common travel rules.

Please answer me these questions in my mind, thank you a lot in advance.

1. 20 March 12 is the date I have to leave Ireland or UK?

2. If I would like to stay 1 week more in UK, what can I do?

3. If I change my flight to departing from UK airport before 20 March, not Dublin, is it ok?

I am a passport holder no needs tourist visa for UK, IE and Schengen area.

Thanks again.

[vinny]

See also Section 2.

[bearteacher.tw]

After reading the section 2, I still can not answer myself about my previous questions.

Please help me to clarify it..

Thanks.

[vinny]

Annex I

[Mr Rusty]

Having entered the Republic of Ireland legally and not requiring a visa to enter the UK, you automatically have permission to stay in the UK as a visitor for 3 months from the date of your arrival in the UK. As you did not see an Immigration Officer on arrival in the UK, it is important that you keep copies of tickets and anything else that confirms your dates of travel in and out of the UK and the Republic.
I don't know for sure, and I don't know which bit of Irish legislation confirms the so-called "Common Travel Area" arrangements, but I would expect that they will class your return to the Republic as a fresh visit, although, again, you may well not see an IO on arrival there. It looks as though they kept a record of your first entry, so you could expect them to get a little bit difficult if your intentions when you return are anything other than a very early departure from Dublin.

[bearteacher.tw]

I realized my crucial point of questions is:

The date written on the ROI entry stamp is fixed or flexible into 3 month?

There are other words printed on the green stamp but I could not read it clearly. It is something about three month. Someone knows the whole paragraph?

Thanks very much.