Most generic mercs in taverns are Christian (this includes the 'Spearmen', 'Watchmen', 'Veterans' and such), whilst the interestingly named ones such as the Svear and the Finnish Archers are (unsurprisingly) pagan. The latter tend to be very good, but also pricey and damaging to morale if mixed with Christians.
In my experience, the best troops have been Svear mercenaries (I've never managed to actually train up peasants, because all the units are at really high levels), but the cots are horrendous.
In your position, I'd just go with Irish faction troops that you recruit from towns owned by friendly lords, and take whatever you can. It's tough and not always practical to get your hands on top-notch troops in VC. I gotta say though, all my Irish characters have sucked badly, but that's probably because I'm a bad shot with bows and slings (which also suck in VC).