The figures, which are published in the latest edition of the Irish Medical Journal, showed that 73pc of mothers in their early 30s took folic acid before conception, compared to 29pc of mothers aged twenty or less.
The researchers said that the findings were "concerning" because folic acid is known to help protect against neural tube defects such as spina bifida.
Ireland has one of the highest rates of neural tube defects in Europe, although this has been in steady decline because of increases in the fortification of some foods and increases in multi-vitamin supplements.
Dr Sinead McNally and Dr Ashling Bourke of the School of Psychology in Trinity College in Dublin said that up to 70pc of neural tube defects can be prevented by taking folic acid before conception and in the first 28 days of pregnancy.
The new study was based on a sample of more than 11,000 women participating in the first round of interviews in the Growing Up In Ireland study.
It found that 64pc of mothers took folic acid prior to becoming pregnant, and that 93pc took folic acid during the first three months of pregnancy.
"Mothers with a higher income, education and age, and those in a relationship were significantly more likely to take folic acid prior to becoming pregnant," the study found.
Folic acid is a B vitamin that is found in some foods as well as in supplement form. All women who could become pregnant are advised to take a supplement of 400 micrograms of folic acid each day.