is íarn the oldest irish word for iron and do we know the origin of the word? What would be correct pronunciation? The same as the modern iarann?

*īsarnom

what is strange is that the Welsh word haearn (Old Breton hoiarn) has an h-, normally Brythonic h- corresponds to a Goidelic s-.

[T]he h- is probably to be explained as arising by metathesis as follows: *isarno- > *iΣarno- > *iharno- > *hii̯arno-; but *iΣarno- > *Σii̯arno- would, of course, also be possible.

Btw, a friend of mine, a scholar, suggested to me that s's could become h's directly without becoming an hypothetic intermediate consonant -- which is possible since Irish s become h's directly when they are lenited.

According to eDIL, the pronunciation was "Orig. ïarn, later íarn, monosyllabic, and íarann". It's pretty clearly from Proto-Celtic *īsarnom with expected loss of medial *s. The ultimate etymology is unknown, although a relationship with PIE *h₁ésh₂r "blood" has been proposed. The Germanic languages may have borrowed the word from Celtic or they may have both borrowed (or inherited) it from a common source.

The only historic source that mentions this deity is a 12th century biography of proselytizing German bishop Otto of Bamberg, who, during his expeditions to convert the pagan tribes of Wendish and Polabian Slavs, encountered festivals in honor of the war-god Gerovit in cities of Wolgast and Havelberg. Gerovit is most likely a German corruption of original Slavic name Jarovit.

Jarilo was a son of the supreme Slavic god of thunder, Perun, his lost, missing, tenth son, born on the last night of February, the festival of Velja Noć (Great Night), the pagan Slavic celebration of the New Year. On the same night, however, Jarilo was stolen from his father and taken to the world of the dead, where he was adopted and raised by Veles, Perun's enemy, Slavic god of the underworld and cattle. The Slavs believed the underworld to be an ever-green world of eternal spring and wet, grassy plains, where Jarilo grew up guarding the cattle of his stepfather. In the mythical geography of ancient Slavs, the land of the dead was assumed to lie across the sea, where migrating birds would fly every winter.

By studying folklore texts from these festivals, and comparing them with the structure of other Indo-European mythologies, the Croatian scholars Radoslav Katičić and Vitomir Belaj reconstructed many ancient Slavic myths revolving around Jarilo. He was a fairly typical life-death-rebirth deity, believed to be (re)born and killed every year. His mythical life cycle followed the yearly life of various wheat plants, from seeding through vegetation to harvest.
Jarilo was a son of the supreme Slavic god of thunder, Perun, his lost, missing, tenth son, born on the last night of February, the festival of Velja Noć (Great Night), the pagan Slavic celebration of the New Year. On the same night, however, Jarilo was stolen from his father and taken to the world of the dead, where he was adopted and raised by Veles, Perun's enemy, Slavic god of the underworld and cattle. The Slavs believed the underworld to be an ever-green world of eternal spring and wet, grassy plains, where Jarilo grew up guarding the cattle of his stepfather. In the mythical geography of ancient Slavs, the land of the dead was assumed to lie across the sea, where migrating birds would fly every winter.
With the advent of spring, Jarilo returned from the otherworld, that is, from across the sea, into the living world, bringing spring and fertility to the land. Spring festivals of Jurjevo/Jarilo that survived in later folklore celebrated his return. Katičić identified a key phrase of ancient mythical texts which described this sacred return of vegetation and fertility as a rhyme hoditi/roditi (to walk/to give birth to), which survived in folk songs:
...Gdje Jura/Jare/Jarilo hodi, tu vam polje rodi...
"...Where Jura/Jare/Jarilo walks, there your field gives birth..."
The first of the gods to notice Jarilo's return to the living world was Morana, a goddess of death and nature, and also a daughter of Perun and Jarilo's twin-sister. The two of them would fall in love and court each other through a series of traditional, established rituals, imitated in various Slavic courting or wedding customs. The divine wedding between brother and sister, two children of the supreme god, was celebrated in a festival of summer solstice, today variously known as Ivanje or Ivan Kupala in the various Slavic countries. This sacred union of Jarilo and Morana, deities of vegetation and of nature, assured abundance, fertility and blessing to the earth, and also brought temporary peace between two major Slavic gods, Perun and Veles, signifying heaven and underworld. Thus, all mythical prerequisites were met for a bountiful and blessed harvest that would come in late summer.
However, since Jarilo's life was ultimately tied to the vegetative cycle of the cereals, after the harvest (which was ritually seen as a murder of crops), Jarilo also met his death. The myth explained this by the fact that he was unfaithful to his wife, and so she (or her father Perun, or his other nine sons, her brothers) kills him in retribution. This rather gruesome death is in fact a ritual sacrifice, and Morana uses parts of Jarilo's body to build herself a new house. This is a mythical metaphor which alludes to rejuvenation of the entire cosmos, a concept fairly similar to that of Scandinavian myth of Ymir, a giant from whose body the gods created the world.
Without her husband, however, Morana turns into a frustrated old hag, a terrible and dangerous goddess of death, frost and upcoming winter, and eventually dies by the end of the year. At the beginning of the next year, both she and Jarilo are born again, and the entire myth starts anew.

Ancient Slavs viewed their world as a huge tree, with the treetop and branches representing the heavenly abode of gods and the world of mortals, whilst the roots represented the underworld. And while Perun, seen as a hawk or eagle sitting on a tallest branch of tree, was believed to be ruler of heaven and living world, Veles, seen as a huge serpent coiling around the roots, was ruling the world of dead. This was actually quite a lovely place, described in folk tales as a green and wet world of grassy plains and eternal spring, where various fantastic creatures dwell and the spirits of deceased watch over Veles' herds of cattle. In more geographical terms, the world of Veles was located, the Slavs believed, "across the sea", and it was there the migrating birds would fly to every winter. In folk tales this land is called Virey or Iriy. Each year, the god of fertility and vegetation, Jarilo, who also dwelt there during winter, would return from across the sea and bring spring into the world of the living.
Veles also regularly sent spirits of the dead into the living world as his heralds. Festivals in honour of him were held near the end of the year, in winter, when time was coming to the very end of world order, chaos was growing stronger, the borders between worlds of living and dead were fading, and ancestral spirits would return amongst the living. This was the ancient pagan celebration of Velja noc (Great Night), the relic of which still persists amongst many Slavic countries in folk customs of Koleda, a kind of combination of carnival and Halloween, which can happen anywhere from Christmas up to end of February. Young men, known as koledari or vucari would dress long coats of sheep's wool and don grotesque masks, roaming around villages in groups and raising a lot of noise. They sang songs saying they travelled a long way, and they are all wet and muddy, an allusion of the wet underworld of Veles from which they came as ghosts of dead. The master of any house they visited would welcome them warmly and presented them with gifts. This is an example of Slavic shamanism, which also indicates Veles was a god of magic and wealth. The gifts given to koledari were probably believed to be passed onto him (which makes him very much like a dragon hoarding treasure), thus ensuring good fortune and wealth for the house and family through entire year. As seen in descriptions from the Primary Chronicle, by angering Veles one would be stricken by diseases.

The historical beginning of the peoples we later know as Illyrians is placed at approximately 1000 BC.[15] The origin of the Illyrians remains a problem for modern prehistorians. The consensus of the primordialists[16] is that the ethno-linguistic ancestors of the Illyrians, labelled Proto-Illyrians, branched off from the main linguistic Proto-Indo-European trunk before the Iron Age. Current theories of Illyrian origin are based on ancient remnants of material culture found in the area, but archaeological remains alone have so far proven insufficient for a definite answer to the question of the Illyrian ethnogenesis.[17]
When the Proto-Illyrians became a distinct group remains unclear. They emerge out of the wider Paleo-Balkans group by the Iron Age, although, since the language is not known in any detail, it is uncertain which populations should be classed as "Illyrian" on ethno-linguistic grounds, and many tribes formerly classed as Illyrian are now considered Venetic.[18]
An autochthonous model, assuming an Illyrian ethnogenesis in the Balkans, was proposed by Alojz Benać and B. Čović, archaeologists from Sarajevo, who hypothesize that during the Bronze Age there was a progressive Illyrianization of peoples dwelling in the lands between the Adriatic and the Sava river. This theory was also proposed and supported by Albanian archaeologists for the southern Illyrian tribes,[19] while Aleksandar Stipčević says that the most convincing model of Illyrian ethnogenesis was that of autochthony, excluding Liburnians.[20]


These tribes, or at least a number of tribes considered "Illyrians proper", are assumed to have been united by a common Illyrian language,[5] of which only small fragments are attested enough to classify it as a branch of Indo-European, while it was extinct by the 5th century.[6]

íarn - iron
iarannach - iron people - Iron armed warriors - people who know how to make iron

Probable origin is from old Serbian word "jara".