Political violence
The IRA, or Irish Republic Army, play an important role in the events of Trespasses. The IRA was a terrorist organization motivated by political beliefs to enact violence across Ireland and the rest of the United Kingdom. In carrying out that violence, they hoped to reunify Ireland and prevent the United Kingdom from influencing Ireland. They killed people, maimed people, and otherwise inspired tremendous chaos around the country for a political cause. In the end, political violence achieved nothing other than the deaths of over 10,000 people, showing that violence done in the name of politics solves nothing.
The intoxicating power of love
The intoxicating power of love is one of the most important themes in Trespasses. The novel's romantic relationship between Cushla and Michael is at the center. Their relationship, in many ways, is unlikely: they come from different religions, are vastly different ages, and Michael is a married man who sympathizes with the terrorist organization called the IRA (or Irish Republican Army). Their relationship was also discouraged by both Cushla and Michael's families, who felt that it was inappropriate.
Nevertheless, Cushla and Michael continued their relationship. After all, they were intoxicated with each other and could not let each other go. They felt that they were soulmates and loved each other tremendously. Love makes people ignore the people around them and the warning signs surrounding their relationship, which ultimately hurts them (just like it hurts Cushla).
Religious conflict
Religious conflict is at the center of Trespasses. After Cushla, the novel's main character meets a man named Michael Agnew, whom she falls deeply in love with and starts a romantic relationship with. Traditionally, people in Ireland entered into romantic relationships with people who shared their religious values. For instance, because Cushla was Catholic, she was expected to court and then marry a Catholic man. However, Michael Agnew was a Protestant man, making their relationship taboo. To make their relationship work, Cushla and Michael had to navigate the conflict between their religious beliefs (and the conflict between the people in their lives who were not happy with the conflict between Cushla's Catholicism and Michael's Protestantism). That conflict reveals something about Ireland's culture and how it fosters naivete and unneeded prejudices. It also shows that people who love each other will find a way to be together no matter their differences (especially their religious differences).