Currently out of print. It is available in some public libraries, and the UCD Architectural Library at Richview has copies. You could also check adverts.ie which has had copies for sale previously.
As Homebond House Building Manual shows best building practice together with summary of Building Regulations, & I understand that the main difference between each edition is updates to the Building Regulations, as ONQ suggests you could consult the Technical Guidance Documents of the Building Regulations on the Dept Of Environment website, and refer to an earlier edition for best building practice.
Hope that is of use.