Am 18 year old boys mum. He has just done his leaving and about to go to UCC.

I imagine it's distressing, but your 18 year old is a man, and can make his own decisions about right, and wrong now, mum.

Obviously it's not wrong for him at this time. How long have they been together?

A 26 year old is not a girl and an 18 year old is no longer a boy.
As a mother I would be upset with this grown woman dating my son.
How long has this been going on?

You can't tell him it's illegal, and if you do tell him it's 'wrong,' (a matter of personal opinion), then you may very easily drive him further into her arms. Speak your mind as quietly as possible to him, and keep it short. He's right on the edge of being old enough to make his own decisions, and young enough to make some really dumb ones. You could try the suffering mother approach of "You may no longer want to listen to the advice of your old mum who spent 18 years trying to teach you a thing or two about life," but it could backfire. Anything you say could.

Yet I don't blame you for wanting to try. Just do your best to keep it to a few sentences.