There are various expenditure measures presented in the most Education at a Glance, and expenditure as a percentage of GDP is just one of them.
An alternative measure of education spending that is also presented in the OECD report is public expenditure on education as a % of total public expenditure. The OECD reports shows that in 2018 this stood at 12.6 per cent, well above the OECD average of 10.7 per cent. Ireland was ranked tenth for this indicator in the OECD report.
It should also be noted that Ireland has consistently argued that, given the well-known measurement issues with GDP due to globalisation effects, an alternative measure should be used. This is acknowledged in the methodological notes of the report which says that in 2016 Ireland produced a modified estimate of the gross national income (GNI*) that was recommended by the Economic Statistics Review Group and which is designed to specifically exclude these effects.