Grainne is a possibly derived from the Old Irish grán meaning "grain" or gráin meaning "hatred, fear". Based upon the spelling I fear it's from gráin meaning "hatred, fear".

It sounds so sweet though: GRA-nyə.

I'll have to do more research! I did read this comment on BtN:

"The claim that Gráinne derives from grán (“grain”) or gráin (“hatred, fear”) originates from a single 19th-century scholarly speculation (likely Whitley Stokes or Kuno Meyer) that has since been dismissed. Modern authorities such as Ó Corráin & Maguire (Irish Names, 1981) state clearly that Gráinne is an ancient Irish name of uncertain origin and meaning. It has no connection to words meaning “hatred” or “fear”, and repeating that association is both linguistically wrong and offensive. Please consider correcting the entry to reflect this."