Pi'neal (pin and Neil not pine) with stress on the i. Like co'rdial and not like ide'al. By the way, it's not pronunciate but pronounce (pronunciation is the noun).

maybe a regional variation in accent, but I always pronounce it "Pie Neil"

Me, too.

I'd only ever pronounce it pin nial, emphasis on the first syllable

and Chambers etymology would seem to support this:

pineal gland and pineal body noun, anatomy in vertebrates: a small outgrowth from the roof of the forebrain, which produces the hormone melatonin. Also called epiphysis.
ETYMOLOGY: 17c: from French pinéal, from Latin from pinea pine cone.

from: http://www.chambersharrap.co.uk/chambers/chre...