Basis for saying a passport is not needed for Northern Ireland

Hello,
The Ireland.com site says it is not necessary to have a passport to visit Northern Ireland provided entry is from the Republic of Ireland. Can you tell me what is the basis for this statement ? When I look at the gov.uk site, it says:
 « Travelling from Ireland to the UK
If you’re from an EEA country or Switzerland
You will not pass through any immigration control when you enter the UK from Ireland across the land border, so you don’t need any documents to enter the UK on that route.
However, you may be asked to show your passport (which should be valid for the whole of your stay) or identity card to enter Great Britain when travelling from Ireland if you are encountered by Border Force. You cannot use an EEA or Swiss national identity card to enter the UK, and can only use a valid passport. »
This is contradictory to the Ireland.com statement.  Could you please clarify/ point me to the official source that is behind the statement on Ireland.com?
Thanks

Hi, Fiona,

Sorry for the delay in my reply.

While this part of the Government website can be confusing, it highlights the same guideline we have on Ireland.com.

Entering Northern Ireland via the Republic of Ireland, there is no immigration control so your ID card is sufficient.

On the other hand, if you are looking to travel to Great Britain from the isle of Ireland, which involves taking a plane or ferry, you will require a passport as travel with an EEA or Swiss national Identity card is not allowed. 

"You cannot use an EEA or Swiss national identity card to enter the UK, and can only use a valid passport" only refers to direct travel to the UK - including Northern Ireland - as there is immigration control when arriving directly to the UK by plane or ferry. Know that there is also exception here, which are listed on the Irish official site.

I hope this helps clarify the information.

Elodie