The range of hypotheses is pretty wide as to these "Igylliones". But we have to start with at least a few more or less stable assumptions. (Assumptions, unfortunately not certainties).
1. It is more likely than not that the Igylliones were not known to either Strabo or Pliny.
2. They would thus represent a reality very close in time to Ptolemy, viz., the 2nd c. CE. This has some support in the contextual sequence Igylliones--Costoboci--Transmontani, and in Igylliones as "below" the sequence Venedae--Galindae--Sudini--Stavani--Alauni.
3. The Igylliones would thus be the population inhabiting the area of Ukraine's Galicia and southern Volynia at that time, viz., the "fusion" of Przeworskers (there since 50 BCE -->) and incoming Zarubinians from the older group 1 area (esp. since 50 CE-->)
One immediate question: which component would have contributed the name for the new whole? More likely than not the Zarubinians (since "Peucini" and "Baste(a)rnae" are also acceptable labels for this group, not only in Pliny and Tacitus, but also in Ptolemy).
So assumption after assumption (:=)) leaves us with the possibility of trying to ferret out a meaning based on our meager knowledge of Bastarnian history... What would apply better: "smiths" or "hedgehogs"?
1. Either, if the name was that of a leader of incoming Zarubinians (Scythian "Skoloti" analogy): "Smith" or "Hedgehog"...
2. Otherwise, other possibilities come to mind. "Hedgehog" might have military implications (though the ones I discovered are very modern...). Still, by analogy: one can think of the phalanx, which is indeed like a "hedgehog". And the Bastarnians may have fought that way, esp. in the Mithradatic wars. All we know about them, really, is that they were considered fierce warriors, and even "the bravest of the brave". Pretty slim I grant. Or even slimmer: when they intermittently occupied Peuce, the island became a sort of impregnable fortress, a "hedgehog" if you will...
As to the Finnic hypothesis. I don't know about that. For material evidence I'll wait for Pachkova. I think that Ptolemy's mention of "Finni" below the Goths is almoist certainly misplaced.