You don't need to register as self-employed.
The UK company needs to register a branch address in Ireland using Form F12
[broken link removed]
http://www.cro.ie/ena/forms_f_to_h.aspx
They also need to register as an employer in Ireland using Form TR2 and pay your wages through the Irish PAYE system.
http://www.revenue.ie/en/tax/vat/forms/formtr2.pdf
You are then paid as a normal Irish employee with Class A PRSI contributions etc etc
The UK company must file a copy of their annual accounts with the CRO every year attached to a Form F7 (see link above)
It seems like a lot of work but once the set-up is done there is very little work really. Any accountant should be more than willing to set it all up for you and allow their address to be used as the registered address in Ireland (for a fee obviously)
I've done this for 3 different UK companies with sales reps in Ireland with no problems at all.