There is a great deal of interest both popularly and amongst scholars about whether cognitive ability is predicted by handedness. The literature contains many findings which cannot be simply summarized and there are many many myths. Evidence for Ireland has been non-existent, as far as I am aware, until now with the release of the Growing Up in Ireland data.
So what can we say? Below I plot the density of attainment at a maths test that the 8 year olds in GUI sat.
Sadly, if you are a ciotóg, you can see the distribution is shifted to the left - but not by much. The good news is that when you look at the distribution of reading ability, there is no difference at all.
In numerical terms, left-handers are about 8% of a standard deviation lower. By comparison girls are about 11.5% of one standard deviation lower.