Niav gave him an enchanted horse on which he could visit his old haunts and estates.The speed of the horse seemed to increase as it galloped through the countryside. It was moving so swiftly that Oisín saw only glimpses of the landscape around him. He noticed a subtle change to the landscape. Where there had been wild and untamed moorland and bog, the haunt of wild animals and birds, there were now bordered and cultivated fields, in which cattle grazed and crops had been planted. Where there had been desolate stretches of waste and rock, there were now clusters of small cottages that sent their smoke curling lazily into the air, and tended gardens, where men worked raising small crops of potatoes. Even the roads seemed somehow changed: where there had been no more than grassy tracks, leading off into the forests or mountains, they now seemed well paved and better laid-out with stones and proper borders. The countryside seemed to have undergone changes that would have taken decades to complete.

Oisín came upon several men who were trying to move a large rock which lay in the middle of the road. An old man sat on a rock by the roadside, watching and directing four younger men as they pushed and strained to remove the obstacle. They had no success and, as he looked at them, Oisín realised that they were smaller and less muscular than the men with whom he had fought in the great days of the Fianna. The men all looked up as he pulled his horse up alongside them.

‘Is this the road to Royal Tara?’ asked Oisín. The young men eyed him warily.
‘It is the road to Tara sure enough’, one of them replied. ‘But I've never heard it called Royal Tara before’. He spoke in an insolent fashion and in a harsh and unfamiliar accent that Oisín didn't like.
‘You know that it is the seat of the High King of Ireland and of his warrior’, he snapped. ‘Is he a madman?’ he heard one of them ask his companion.

‘There has been no High King at Tara for countless years!’ He spoke in the same strange accent as the first speaker and there was a general murmur of consent among the men gathered by the roadside. Suddenly afraid, Oisín felt for his sword and said:
‘Have a care whom you would call mad! For I am Oisín, son of the mighty warrior Fionn MacCumhaill, hero of the Fianna’. At his words, the men broke into scathing laughter.
‘Now I know that you're mad’ he taunted. ‘Oisín is only a figure of myth, an old legend who was supposed to have vanished into the Otherworld over three hundred years ago. Songs about him are all but forgotten and it is even said that he never existed at all. You can't be him!’
‘No!’ the voice came from the old man seated on the roadside rock. ‘Oisín is no myth. He was a man who lived almost four hundred years ago and who was lured away by some enchantress into the world of the Sidhe. It was widely prophesied in my great-grandfather's time that he would, one day, return and that no one would know him. Tell me great warrior, are you that hero?’

Oisín's mind reeled from what he had been told. And yet, such a thing would explain the changes that he'd seen as he hurtled through the Irish countryside on his way here.

‘You lie, old man,’ Oisín spat. ‘How can nearly four hundred years have passed, when I know that I've spent only a few days in the Otherworld?’
‘But time in the Otherworld passes differently from that in the mortal world,’ he said slowly. ‘A day there may count for a hundred years in this. But you haven't answered my question sir. Are you indeed the Oisín of whom heroic ballads are still sung?’
Oisín struggled to reply. ‘I . I am!' he stuttered. ‘And I'll prove it to you. I'll move this rock that you seem to find difficult. Only a true warrior of the Fianna could move it without dismounting’.

Leaning down from the saddle of the horse, he took a grip on the stone and began to move it. The young men watched in astonishment. Slowly, and with some difficulty, the stone was eased to the side of the roadway. Strangely, Oisín found the rock heavier than he had imagined it would be – perhaps his famed warrior-strength had diminished a little in the Otherworld. As he leant forward to get a grip on the sides of the stone, the strap that secured his saddle snapped. At the same time, the snorting steed moved forward a little and Oisín found himself tumbling to the ground. Instinctively, he thrust out a foot to save himself, touching the ground only for an instant. The moment the sole of his boot touched the earth of the human world, Oisín felt a tremor run through his entire body, and remembered what Niav had said: ‘Do not set your foot on the ground Oisín or you will be lost, both to this world and your own’.

The others stared at him in wonder, as Oisín, son of Fionn MacCumhaill and mighty hero of the Fianna aged before their eyes. In a moment, he had become an old man with shaggy hair and a white beard, fearfully bowed over and marked with all the signs of a long and careworn life. Even then the change was not complete, and as they watched, the ancient creature crumbled into a fine, grey dust that began to blow away in the wind.

From Complete Guide to Celtic Mythology by Bob Curran