In the Middle Ages, a scholar should be aware of The Matter of Britain, The Matter of France, and The Mattter of Rome. That is, Arthurian legends, the tales of the epic cycles, and Greek mythology as filtered thru Roman (including epics like the Iliad, Odyssey, and Aeneiad). In the early medieval, it was assumed anyone in northern Europe who knew Greek was Irish! It remained an arcane language until the Renaissance.

Love celtic history! Not a scholar per se, but the short answer is yes. (for purpose of citing source, check out this awesome documentary by the BBC that goes into great depth https://www.youtube.com/watch?v=AU1dKfMIEUQ; I got most of my information here.)

First off, defining 'viking era' and 'medieval era' yield different time frames. Medieval Ireland and England would have been dominated by the Catholic influence from Rome, thereby misplacing the timeline here. I think for this case lets look at the (very broad) Iron Age era (800BC-100AD).

Another issue is the definition of Celtic. Contrary to popular belief, the people who inhabit and are inhabiting the island of Ireland were not exactly totally 100% genetically Celtic. The Celtic people as we know them originated throughout central europe and Spain. Most historians would classify the Irish at that time as very muchculturaly celtic, but not so much kin. Kinda like how Americans are culturally American, but genetically not so much. In fact Ireland was popular trading outpost that had great diversity during the late Iron age and into the medieval era.

But to answer your question, Yes! Greek cultural in fact dominated celtic culture in the beginning and continued its effects later on. Many celts had greek pottery and symbolism, and used common elements to combine into celtic culture. Greece at the time was considered the cultural epicenter and it was very much in vogue to know what the Greeks were doing.

Forgive the lack of examples, but your question is well founded and actually opens up a whole mess of other questions for culture identity and heredity that has interested me about my Irish family and roots! The world I find is truly a melting pot...

Yes, Irish writers and chroniclers sometimes alluded to Roman and Greek literary and historical figures. They didn't compare their indigenous and Greek mythologies, but they did compare living historical figures to Graeco-Roman heroes: Muirchertach mac Néill, a 10th century king of Ailech, was eulogized as the "Hector of the western world" in the Annals of Ulster, for example. It is entirely likely that Irish scholars would have been familiar with classical Graeco-Roman deities but I've never come across a mention of them in primary sources before, likely because they were too pagan to be discussed by Christian monks, however human heroes seem to have been fair game. For the same reasons, historical texts rarely mention pre-Christian Irish deities (with exceptions) but do describe the deeds of pagan Irish kings like Niall Noígíallach. Allusions to Hebrew and "eastern" characters are actually much more common as the Irish believed that their language and people began in the Biblical near-East and migrated to Ireland across Scythia, Germania, Gaul and Iberia. According to the narrative of the Chronicon Scotorum, the first humans to arrive in Ireland were a Greek princess named "hEriu" (after whom the island was named) and her entourage, showing the perceived close links between Ireland and the eastern Mediterranean. Interestingly, some early medieval Irish annals mark the deaths and coronations of contemporaneous Byzantine emperors.