Prior to taking anyone on in Ireland, rather than hiring mercs to make up the difference, use what gold you have to upgrade your counties (by which I mean the castles you personally control, not the cities or churches). Wooden palisade adds tax revenue, garrison/levy size, and fort level, and only costs 50 gold. Militia Training (I think) adds something like 100 troops (light infantry and archers) to a castle's levy (very helpful for bulking up your numbers), and costs 100 gold. If you want it to go faster (the wooden palisade takes at least a year, maybe more?) use your steward to oversee construction in that province (note: this only effects, specifically, holdings in a county that belong to you, not your barons/mayors/bishops).