There are 2 pay rates - pre '95 and post '95 - and within each of those, a standard and higher scale.
The OP mentioned HEO, so [broken link removed] the link to the PSEU pay rates for that grade.
As an example the Point 1 on each scale for HEO is;
Pre '95 Standard €45,422
Pre '95 Higher €48,175
Post '95 Standard €47,812
Post '95 Higher €50,712
I've only worked in 2 different departments, but both of those assigned people to the higher scales on a competetitive basis, similar to a competetition for promotion (but scaled down).