There certainly was Irish raiding in Britain in the fourth and fifth centuries and this may well have continued later into the period. The first attestation relates to the 360s, and comes from the Roman historian Ammianus Marcellinus. The poet Claudian claims that Irish raiding remained a problem c. 400, though his desire to flatter his subjects may have caused him to overemphasise the threat. Nevertheless, some level of raiding certainly did take place. Our best source for this is St Patrick's Confessio, which is essentially an autobiography. As a youth, Patrick himself was taken as a slave by Irish raiders. Gildas, also a Briton, claimed that Irish raids devastated much of (what had been) Roman Britain in the earlier half of the fifth century, though it's likely that he exaggerated their extent.

Indeed, the extent of Irish raiding is very difficult to ascertain, since this kind of small-scale warfare rarely leaves archaeological evidence. It might be worth pointing out that the Romans didn't construct a series of coastal fortifications on the west coast, as they did on the east coast. Then again, it could simply be that the west's poverty meant that it just wasn't worth defending.

Like the Vikings, the Irish didn't just sail to raid; they also sailed to trade and settle. Ogham, a Celtic alphabet, was used on both sides of the Irish Sea, and some British inscriptions are carved alongside a Latin inscription, indicating peaceful contact between the Irish and Romano-Britons. As with the Vikings, some Irish settlers became rulers in Britain. Dál Riata was an Irish kingdom in western Scotland, and the Welsh kingdom of Brycheiniog also probably had Irish rulers.

There's no evidence of Irish raiding further afield than Britain, which makes sense given that their boats weren't suited to longer sea voyages. Patrick's Epistola informs us that British raiders were coming to Ireland to take slaves, sometimes selling them to other Irish people, so raiding was happening both ways.