For sure they are related – just that one is Celtic and the other is Serbian. I am sure Crom Dhub is as old as when the Celts were in the Balkans.

Check it out!

"Crom" is a Celtic term meaning "bent".
"Hrom" is a Serbian term meaning "lame".

Both are dark gods: Celts say he was a dark, evil god; Serbs say he was an ugly god of the underworld.

Crom Dubh or Crum Dubh (Irish pronunciation: [krˠoumˠ d̪ˠuβˠ], Scottish Gaelic: [kʰɾɔum t̪uh]), meaning "black crooked [one]", alt. "Dark Crom", was a Celtic god, for which see The Voyage of Bran, Book II. He may have been represented by megaliths.

Dé Domhnaigh Crum-Dubh – "Crom Dubh Sunday" – is known in Ireland as the first Sunday in August, but in Lochaber is applied to Easter. It appears in the Scottish saying:
DiDòmhnaich Crum Dubh, plaoisgidh mi an t-ugh."Crooked black Sunday, I’ll shell the egg."
The exact origin of this saying is unknown, but there is some evidence that Crom Dubh was a fertility god. In later times, he came to be considered an evil god as Christianity spread through Europe as part of the suppression by Christians of the worship of Pagan deities. The element "dubh" (black, dark) had sinister connotations in Christianity, this also perhaps leading to a large part of the eventual association.

Of particular interest is the Serbian Dabog or Dajbog (most modern mythographers take for granted this is the same character as East Slavic Dažbog/Dažboh), also known as Hromi Daba (meaning "Daba the Lame"), described in folklore as a lame "shepherd of wolves", an ugly demon-lord who rules the underworld and travels through the world of men. Though not always evil in folk stories, Dabog/Dažboh is often presented as an enemy of the Christian God or heavenly saints. While this is consistent with the satanization which the Slavic solar deity underwent after Christianization, there are some pretty good indications that even the original, pre-Christian Dabog/Daboh or Dažbog/Dažboh had very strong chthonic attributes