It is generally known among learners that emotions and feelings (and illnesses) are in Irish on (ar) the person experiencing them: Tá áthas orm. Tá lúcháir orm. Tá brón orm. Tá fearg orm.
However, love and hatred are in Irish not perceived as transient emotions, but as attitudes. Those are in Irish something you have, i.e. you use the preposition ag ‘at’. If you use ar with these nouns, it does not refer to the person who does the loving or hating, but to the person or thing loved or hated, although in this sense the preposition do is more common.
The correct construction is, thus:
tá grá ag Seán do/ar Ghobnait ‘Seán loves Gobnait’
tá fuath ag na páistí don scoil/ar an scoil ‘the children hate the school’ (by the way, this can also mean ‘children hate school’, noting that the definite article also can have a generic sense in Irish).