OK, long time lurker here with his niche topic finally being mentioned :)

The main benefits to the crown at the time included firstly a huge protestant and loyalist/unionist majority for centuries to come (starting to become the minority at the present time). This gave them a number of benefits mainly including the increased taxes from another relatively prosperous province. The population of their native countries where relatively happy as they had a new source of land that was not in the Americas. This meant that Ulster was quite popular due tot he fact that it only would take a day or even a few hours to cross to in comparison to other English possessions such as Virginia or Massachusetts.

Another large part of the benefit was the fact that the monarchy was perpetually worried about a foreign, probably Roman Catholic power (primarily Spain at this time) using Ireland as a 'stepping stone' for the main invasion of Britain. This proved to be a good idea as both the Spanish monarchy and Napoleon later considered this plan.Ireland had to be swept into the arms of England/Britain essentially to keep the latter nation safe. This policy can be traced back to 1169 when King Henry II of England mounted his first invasion of Ireland. Neither Henry nor his successors succeeded in gaining control of all of the island.

It was also a popular policy for the nobility, large scale merchants and guild members as they were all given large plots of land after the plantation. The London guilds were notably given land in the north west a county still known (rather contentiously due to the English connotation) as London-Derry. A number of both small and large scale castles were built and inherited from the previous residents, notably Enniskillen castle once a stronghold of house Maguire and the newly constructed castles in Crom, south Fermanagh.

Sources: http://www.sciencedirect.com/science/article/pii/S2212682112000297