It is not entirely clear what is meant by the name of him you may not reveal to any person, that men may not swear by him. For example, it is plain that the location of the tomb is involved in the swearing, not just the knowledge of the person's existence/name, otherwise hiding the body would be pointless, however, there is no indication of why this would be the case. A grisly example that may be indicative can be found in the 12th C. Life of St. Cadog* (English Translation):

Long ago the same patron of revered memory built an elegant monastery of stone work in Albania, Scotland, on this side of the mountain Bannauc. Brethren having gathered therein, he appointed that devout service to God should be given by them for ever. In a certain porch of this monastery the bodies of three of his disciples are lying buried in marble sepulchres. But no one dares to inspect the sarcophagi of the same except a bachelor or virgin, or one ordained. But there is a certain opening outside in the wall of the porch, through which kings and magnates of that country, if by chance a matter of great dissension has arisen among them, put in their hands and perform the solemnities of an oath. If any one breaks that oath, he will go down to death before the end of that year. Now a multitude of people was gathered together in wonted manner on the day of the festival of saint Cadog to hear mass. When the celebration of the mass was ended, a certain foolish rustic, noisy with loud clamour in the midst of the peoples, said to the presbyters, ‘Will you let me go to the opening that I may look through it?’ Replying, they said to him, ‘Go, and may saint Cadog cause that a mark of vengeance appear on thee.’ The rash fellow, therefore, ran to the opening, and covered one eye with his hand placed thereon, for with the other open he looked through the window, and sooner than said the open eye cracked, and by the optic nerve hung down his face. So thereupon the rustic, emitting a great and mournful lamentation, hurried in mad course towards the crowds of people. When he was seen, the whole multitude with uplifted voices poured forth praises to God and saint Cadog, saying, ‘From the rising of the sun and the setting the name of the Lord is to be praised.’ Indeed the rustic likewise went about from place to place throughout the whole province of Lintheamina, covering his torn eye. Many used to bestow a reward on him, that he might show them the rent orb of the eye. Therefrom his compatriots were learning to fear God more and more, and reverently to glorify him with his saint.

This not only has oaths sworn on a holy tomb, but a taboo on viewing the tomb as well. The swearing on the body of a saint may be what is indicated in the entry for 1106 (actually 1109) in the Brut Y Tywysogion (from the translation by Williams ap Ithel*: this conflates several sources**, so the manuscript this is from is not clear, but it is not in the Peniarth 20 versions*):

And the Rickart the steward sent to Madog, desiring him to seize the men who had done injury against the king; and he objected to it, and did not seize them. And thus criminal, he knew not what he could do, other than seek the friendship of Owain, son of Cadwgan; and this he obtained; and so peace was made between those who before were enemies. And they mutually pledged upon the relics that neither should be reconciled to the king without the other, and that neither would betray the other

[The relevant Welsh is ac ymaruoll vch benn creireu a6naethant hyt na hedychei vn ar brenhin heb y gilyd*]

And again, from 1198:

As for one of them, namely Aberteivi, Maelgwn swore upon several relics, in the presence of monks, afer taking hostages for peace from Gruffudd, that he would deliver up the castle and hostages together to Gruffudd on a fixed day. And that oath he disregarded, giving up neither the castle nor the hostages; divine power, nevertheless, set the hostages free from the prison of Gwenwynwyn.

[...a tyga6d Maelg6n uch benn amryvaelon greireu ygg6yd myneich 6edy kymryt gwystlon y gan Rufud...*; the Peniarth 20 versions translation by Jones* differs only sentence order].

The most famous example of this is Harold Godwinson swearing an apparent oath of allegiance to William of Normandy on relics in 1064. This is captured in the Bayeux tapestry here.

Of course, "relics" need not have been human remains; in the 15th C. Leabhar Breac Life of St Patrick* (Translations), we find that at Nemthur people swear on the flagstone ("Saint Patrick's Rock") on which the saint was born (or from which he preached), and if people swear a false oath while touching it, it sheds water as if crying about the falsehood. Less supernaturally, the Annals of Ulster record Donnchad ua Eochada, king of Ulaid making an oath on the "[St.Patrick's] Staff of Jesus and other relics" in 1101*.

However, generally it is plain that the "Celtic Saints" had an ambiguous relationship with the tombs of the sanctified. The relics of the dead were purposely distributed, so that their votive and healing powers would inspire faith: for example in the 7th C. Life of St Eligius* (Translation) we find that his specialism is the locating of previous saints' bodies which he raided for healing relics. While he also identified sites of false veneration, he was no where near as circumspect as the earlier St Martin of Tours (Translation of 4th C. Life*) who had to face more Pagan sites:

BUT let me proceed to a description of other excellences which Martin displayed as a bishop. There was, not far from the town, a place very close to the monastery, which a false human opinion had consecrated, on the supposition that some martyrs had been buried together there. For it was also believed that an altar had been placed there by former bishops. But Martin, not inclined to give a hasty belief to things uncertain, often asked from those who were his elders, whether among the presbyters or clerics, that the name of the martyr, or the time when he suffered, should be made known to him. He did so, he said, because he had great scruples on these points, inasmuch as no steady tradition respecting them had come down from antiquity. Having, therefore, for a time kept away from the place, by no means wishing to lessen the religious veneration with which it was regarded, because he was as yet uncertain, but, at the same time not lending his authority to the opinion of the multitude, lest a mere superstition should obtain a firmer footing, he one day went out to the place, taking a few brethren with him as companions. There standing above the very sepulchre, Martin prayed to the Lord that he would reveal, who the man in question was, and what was his character or desert. Next turning to the left-hand side, he sees standing very near a shade of a mean and cruel appearance. Martin commands him to tell his name and character. Upon this, he declares his name, and confesses his guilt. He says that he had been a robber, and that he was beheaded on account of his crimes; that he had been honored simply by an error of the multitude; that he had nothing in common with the martyrs, since glory was their portion, while punishment exacted its penalties from him. Those who stood by heard, in a wonderful way, the voice of the speaker, but they beheld no person. Then Martin made known what he had seen, and ordered the altar which had been there to be removed, and thus he delivered the people from the error of that superstition.

A similar event occurs in the Lives of the equally vanguard St Patrick*. The remarkable thing about both cases is the notion that the ghost of the man should be available to talk to the saint at the tomb, and that he has to ask the ghost of his character - implying that the good dead, awaiting resurrection, could be called to conversation as easily as the bad. It is noteworthy that, despite the large number of Pagan stone-worshiping sites destroyed by the "Celtic saints", in each case of identification the distinction is between the respect due the good and bad dead who may be buried at a location - the idea of a Christianized community worshiping stones in a more abstract sense, or worshiping deities (Pagan or otherwise) through the stones, is not called upon. For example, there is no notion of the semitic god being embodied within the rock or represented by it in any of its three forms. Despite this, the person in the wonder, who would be of the good dead, plainly believed the practice to be wrong. It is unclear whether this was from fear that it was a Pagan-influenced ritual or simple modesty.

NB: Of course, in all the above cases, these references were written later than the saints, and may reflect more the time they were written rather than the time which they are supposed to represent.