Here are the additional verses I know:

Oh the Summer time is fading
and the leaves are fiery burning
And the wild mountain thyme
casts its seed 'round the blooming heather
etc....

When the summer's but a memory
And the trees are bare and withered
Still the wild mountain thyme
Sleeps beneath the snow-capped heather

My true loves's waiting for me
Where the summer lasts forever
And I'll always see her smile
As I walk thru the blooming heather
^^