I have to answer your question while disagreeing with its core premise: there is no evidence of a migration to Ireland by a Continental people during the time period that the Classical Greeks and Romans speak about their Northern neighbours whom they labelled as Celts. While Ireland certainly becomes more well linked with continental Europe particularly in terms of Elite material culture, it has not been the opinion of Iron Age archaeologists that there was a migration of Celts (if that word should even be used) to Ireland since the 1990s.

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With that out of the way, there are a number of barriers to an investigation of seafaring in the Irish and British isles in the time period your question deals with: The most obvious is that boats, until very recently, were almost entirely composed of organic material; wooden planks, obviously, but also wool or plant fibre woven into ropes and sails, leather bindings and so on. Some smaller craft could be made of leather stretched over a wood frame. All these materials love to decompose, particularly when in contact with water for a long time, unless preserved in very specific environmental conditions. The second is that the sea is difficult to investigate archaeologically, and the Atlantic and Irish seas would have been all to happy to sweep away a disused boat at the end of its use-life. This, combined with a lack of writing from Ireland in the period you're interested in (Roughly 700 BC - AD 400) and very very little writing about Ireland by other people like the Romans, means we only have a few sources of evidence from Ireland about seafaring in this period.

We do have preserved boats from inland waterways, such as the Lee's island canoe, which was discovered preserved in Lough Corrib due to its having become buried in the silt at the lake bottom. This was a rather small craft, and probably would not have been much use in the open sea.

Our best piece of evidence for a large seafaring vessel is the Broighter boat: a small (18cm long) model of a boat made from gold that was discovered in 1896 in a hoard of other gold objects. The model boat has a mast, yardarm, 9 benches and two rows of 9 oars each, a steering paddle, and within it were models (also in gold) of grappling forks, likely for catching thrown rope and maneuvering in and around docks. If the vessel was made to scale its real-life counterpart is estimated to have been 15 metres long. The other objects in the hoard date it to roughly the 1st century BC, and we know that this was a time of regular contact between Ireland, Britain, and further afield on the continent.

That does it really for Irish Iron Age boats, I'm afraid that its a subject that, barring another big discovery, doesn't have a lot of source material to work with. I hope this helps