Unitarians disagree with the majority of Christians (both Catholic and Protestant) in that they do not believe in the Holy Trinity (Father, Son, and Holy Spirit), but rather one united God. They tend to be more liberal and open-minded in their interpretations of the Bible, especially when it comes to Jesus Christ's divinity (or lack thereof).
Given that an Irish immigrant would almost certainly be staunchly Irish-Catholic, and a poor hill farmer would not be likely to hear of this radical new religion, it comes down to the two groups with money. With the ability to travel and trade, they are more likely to meet new people and exchange new ideas. However, western Massachussetts was quite rural in the early 1800s and even a very well-to-do farmer would not have spent too much time away from his farm. Thus, a merchant in a port city would be most likely to encounter unitarianism, if not simply because their presence in an urban environment would have increased their chances of meeting a Unitarian in the first place.