Most likely nothing, as many Vikings in the early 9th century were only going on Viking trips as a seasonal, temporary expedition, rather than a permanent way of life. To be a successful Viking, you do not need a helmet. Helmets are more a thing for professional warriors, people with fighting as a way of life. This is more likely for a Viking army of the late 9th century.

However, there were certainly some Vikings in 830s Ireland with helmets. Most helmets that re-enactors use are based on slightly earlier models, primarily those of Sutton Hoo or Vendel. These are graves of wealthy people, hardly the average guy, and being a warrior (and thus having professional wargear, like a helmet) was part of what they wanted to show when they died. These kind of helmets, of the earlier period, are usually quite ornate and rich in symbolism. The other period that we have many helmets for is the 11th century, where helmets are much more common as part of the equipment of semi-professional soldiers. These helmets are much more functional. There are some perfect bridges between the earlier ornate helmet, and the later simple functional helmet, such as the Coppergate helmet from York (slightly too early) or the Gjermundbu helmet from Norway (slightly later). In any case, helmets were likely variations of the Roman-period spangenhelm type, with or without cheek- or faceguards, and with some kind of moderate crest.