Greg Not sure if this helps or confuses...
From the Irish:Taken from an online dictionary)

Darach
nm. g.v. -aich; pl. daraich, oak
Darag
nf. g.d. -aig; pl.+an, oak tree

And of what I did know already

The County of Derry in Ireland is known as the oak county, yet the county of Kildare is an anglicised from 'Cill Dara' or the 'hill of Oak'
County Mayo is named from the Yew tree (maigh-eo) Eo being Yew (though check the spelling here...

rgds

Diesel