I've been away this week and would have posted sooner but it may depend on your grid set-up.
A number of the mapping options have different base grids and can give variations of up to a couple of hundred metres. The standard for the UK is 'British Grid' in the position format option, and 'Ord Srvy GB' in Map Datum. Have a look in your Map Datum options and you should have the option to change it to "Irish Grid'.

Hope this helps

Rupert