Shan,

It sounds like the oil tank is yours so I'm assuming it is.

With a howling north wind and concerns about undermining the tank with tree
roots etc. it is difficult to think of an evergreen screen tall enough to
obscure the sight of the tank.

However, if it cannot be screened, have you thought of hiding it?

Hide it under some plant which scrambles all over it.

I suggest you cover the tank with some durable and strong plastic netting (I
can enviseage using some polypropylene fishing net) then plant and train
some creeping plants to cover it.

There are many experts on the ng who can suggest plants which might do well
in Ireland but Roman vine comes to mind for speed! Then there's a few
evergreen clematis which someone must be able to recommend but whatever
plant, I think it needs to be evergreen.

Hope this helps.

Regards

Geoff