I don't like the look of that - it looks like a possible case of one of the phytophthera infections, either P. ramorum or P. kernoviae, often commonly known as Sudden Oak Death. Despite its common name, it does affect other plants, primarily rhododendron, but also Viburnum. Depending which bit of Ireland you live in, these diseases are notifiable if you suspect that's what's going on, more information here https://www.rhs.org.uk/advice/profile?PID=329

I have this planted beside a rhododendron and it doesn't show any signs of the disease. Anyone any other ideas?

I've added another couple of photos.

Anyone?!

I have to admit that I have trouble believing that that has been in the ground for three years. Between the size, and the pristine tag, I'm wondering if you are thinking of a different plant which was planted three years ago. It looks like drought stress to me, which could be as much from root damage as from lack of water. When did the damage first show up, Gerard? When it was planted, was there a layer of compost left under the roots? Was it planted deeper than in the original pot?

Yep definitely planted 3 years ago along with the other plants in the boarder. Can't believe that is has lack of water...I live in Ireland!! It did have soil under the roots but perhaps not enough. Maybe it is competing with the rhododendron which is thriving. Damage showed up 4/5 weeks ago. It wasn't planted deeper than the pot.

What do you mean when you say 'it did have soil under the roots but perhaps not enough'? Is there a hard surface beneath the border it's planted in?

I'm sorry about my doubts, Gerard! I work at a garden center, and customers will show me a picture of an obviously brand new plant, and swear that they planted it 5 years ago...until their wife speaks up, and says, "No, honey, that's the one in the back yard. We bought this one last month." Spouses and partners have eidetic memories, you know. ;)
Since the Rhodie is doing so well, maybe the soil is too acid for the Viburnum, and a cautious application of lime, just around the Viburnum, is in order.

Yes the boarder has hardcore under the soil. It was used to be a driveway around the house. I'm starting to wonder if the vibernum has enough soil under.

What size does this viburnum grow to?

1.5 metres in height - and if the soil only goes down an inch or two below the rootball across the whole border, that's not enough - needs to be a minimum of a spade's depth at least, so that's around 8 inches, but better if its a foot to 18 inches deep. Plus that hardcore may be causing drainage problems ...

Ah, the hardcore is explanation enough. The Rhodie must be a very dwarf variety to be doing so well, there.