A good description on the problem can be found here:
http://www.esri-ireland.ie/software/arcgis/free-mapping-software/ig-itm
The downloadable zip file contains a ntv2 grid for the datum shift.
Official information can be obtained from http://www.osi.ie/Services/GPS-Services/Co-ordinate-Converter.aspx and http://www.osi.ie/Services/GPS-Services/Migrating-to-ITM.aspx and
Both Irish and OSGB datum shifts can be made with ntv2 datum shift grids, but the OSGB only covers England, Wales and Scotland with limited offshore coverage, while the Irish grid has values for the whole rectangle:
