Rosamunde the Rose-mouth survived as Rosmerta.In the last will of the deceased mother, Rosamunde was named first, Minerva second, and Syrhed third in line of sucession. [...] Secretly, she [Syrhed-Kelta] invited Magyars [Finns] to come and teach her sorcery, and when she had had enough of that, she threw herself into the arms of the Gools [Galli]. [...] In those times, Rosamunde — that is, ‘Rose-mouth’— was mother. [...]Kelta, who allegedly could walk as easily on water as on land, went to the mainland and on to Massilia. [...] From this burg she [Syrhed-Kelta] ruled like a legitimate mother, not for, but over her followers, who thenceforth named themselves Kelts.
Minerva Nyhellenia gave us the Minerva or Pallas Athene, but more specifically Nehalennia.
Syrhed-Kelta the Germanic proto-Celt I do not know of, though her name is ancient Germanic Freyjan (Fryan) name Syr or Syritha (source, more here, name Syrhed ↔ name Syritha). The story about her studying sorcery under the Finns survived in the Celtic lore about the Tuatha Dé Danann or aes sídhe studying sorcery in northern islands:
In context needs to be noted that OL, Norse sagas and other sources (including ancient DNA evidence e.g. "Finnish-like ancestry in the westernmost fringes of Europe [...] defying modern historical groupings") puts Finns much closer to Frisians into western Germany (e.g. OL MS 001), Danish islands (Fundinn Noregr) and Scandinavia (Frá Fornjóti) than is apparent from modern-day maps. The main Finnish magus battling Wodin is a Sweden-Finnish king Gylfa in Norse saga accounts (Ynglingasaga ch. 5, Frá Fornjóti). Thus there is no contradiction with allegation of Finnish influence in Danish-Scandinavian geographical locales.304. Thereafter the progeny of Bethach s. Iarbonel the Soothsayer s. Nemed were in the northern islands of the world, learning druidry and knowledge and prophecy and magic, till they were expert in the arts of pagan cunning.
305. There were four cities in which they were acquiring knowledge and science and diabolism: these are their names, Failias [Falun, Sweden], Goirias [Górr's island dominions?], Findias [Fyn, Denmark or Finland], Muirias [Mora, Sweden]. From Failias was brought the Lia Fail which is in Temair, and which used to utter a cry under every king that should take Ireland. From Goirias was brought the spear which Lug had: battle would never go against him who had it in hand. From Findias was brought the sword of Nuadu: no man would escape from it; when it was drawn from its battle-scabbard, there was no resisting it. From Muirias was brought the cauldron of The Dagda ; no company would go from it unsatisfied. There were four sages in those cities: Morfesa, who was in Failias, Esrus in Goirias, Usicias in Findias, Semias in Muirias. Those are the four poets, with whom the Tuatha De Danann acquired knowledge and science.
306. So that they were the Tuatha De Danand who came to Ireland. [Or: Thereafter the Tuatha De Danann came into Ireland.]
[Source: Lebor gabála Érenn IV, ch. 304-306]
Fornjót kings and other Norse saga characters survived into later Christian medieval and renaissance era demon lore where the old kings were turned into spirit or demon lords e.g. Fornjót ↔ Forneus/Formione, Väinämöinen ↔ Vine, Búre ↔ Buer, Guse ↔ Gusion etc. One of these characters is Morax or the Mora/Muirias above, representing another echo of the same core story. Most famous sorcerer or druid was of course druid Jesus who famously walked on water. While not the main topic here, it needs to be noted here in context that there are Nordic and papal traditions about Jesus being of Finnish roots (source retold by an American reteller, Norse saga echo based on same Jesus' adventures in India, pope Innocent III on the same story, see image here). This can perhaps again suggest larger backdrop to such feat in OL and Jesus narratives plus that pre-Celtic Tuatha Dé Danann or aes sídhe ↔ OL Kelta Frisians with perhaps the names based on the story of Frisians as an offshoot of Dan line via Aser or Æsir (more here).