From what we understand, Irish nationals are within the common travel area, do not have any restrictions on residence and should be eligible in the same manner as UK citizens, ie, have to show habitual residence in Ireland or the UK but nothing else.
Is this correct and if so, could someone please point me to the relevant immigration regs or any other source material.

Not sure of the regs - check CPAG handbook.

You are right - our understanding is that Irish Citizens do have an automatic right to reside and, if travelling within the CTA ( i.e from Ireland to UK ) do not have to satisfy HRT. HRT only comes into it if they arrive in UK from outside CTA.

SS (PFA) Amendment regs 2006.

Starting with IS regs 1987, 2006 regs (via reg 6) amends reg 21, by inserting 21AA, which gives the meaning of "person from abroad" and provides that that person is not habitually residence, in UK,Chanel Island, Isle of Man, and the Republic of Irland, if he does not have Right to Reside because.... etc etc. Meaning for all the CTA guys, hbres is required, but not the right of residence.

The 2006 has also amended the rest of the means tested benefits, jsa,pc,hb,ctb etc exactly the same way.

Hope this is ok.

The point is that if someone is habitually resident within the CTA then he is treated as hab res as soon as he arrives in GB, but he does need to establish that he is hab res somewhere in the CTA (ie he can't arrive from Bulgaria in Dublin and then a week later turn up in the UK). But anyone from the CTA has always had a right to reside in the UK and so this ahsn't changed.

I met another of those cases today where the DM didn't know that Ireland is in the EU - where do they find these people???