It's a mixture of Old, middle and modern depending on the term used. Also there are a few typos within (eg. Banda for Banba - a name of Ireland) and there are some not entirely-one-the-spot translations of terms, but overall it's a good 'dictionary'.
In the main the spelling conventions are modern Irish - eg. indicating when a consonant is lenited (softened) by writing h after it. But some words are not modern Irish and have fallen out of use as they belong to a much older period.