Missed Opportunities
Throughout the play, Gar feels unable to seize the opportunity to speak to his father about his emotions and connect with him before he goes to live in America. While Gar is exceedingly frustrated with the silence that exists between him and his father, he is unable to be vulnerable and make the first comment that might open up a channel of communication. When he finally opens up slightly towards the end of the play, he runs away the second S.B. shows any sign of indifference.
S.B. also longs to connect with his son, which is evident in the way he nostalgically reminisces about Gar's childhood with Madge, but similarly to Gar, he is unable to find a way to express himself. According to Gar, it's because they "embarrass each other," however it could also be seen simply as they grew up never showing affection towards each other. Additionally, Gar misses his opportunity to propose to Kate Doogan. Once faced with the threat of a more "eligible" rival taking his place, he immediately flees to escape a situation that he thinks will end up embarrassing him, even though Kate's father suggests that he would approve of their engagement.
Master Boyle is yet another character who appears to have missed many opportunities. He once dated Maire, Gar's mother, before S.B. came in the picture, and seems to feel remorse that they never got together. These missed opportunities that mark the play give it a tragic and melancholic quality.
Traditional Ireland vs. Modern America
The contrast between the more conservative, religious Ireland and the contemporary, liberal America is a major theme in the play. Gar is enmeshed in Irish Catholicism, saying a rosary with his father and Madge at night. However, part of the allure of America is the fact that it is a more permissive and secular nation. Gar has many fantasies about the liberated lifestyle he will have in America, the fact that he will seduce many beautiful women and live a glamorous bachelor's life. His images of America's virtues are informed by Hollywood fantasy.
Additionally, the fantasy of a new life in America comes with a fantasy about more economic freedom. While in Ballybeg, Gar is relegated to working in his father's general shop and cannot seem to get out from under this condition financially, America offers the promise of economic mobility, starting with a job at a hotel. When his Aunt Lizzy comes to visit, she speaks glowingly of all the comforts and space they have in America, and the fact that life is easier, more comfortable, and more financially secure. In the play, America is presented as modern, materialistic, and liberal, in contrast to the conservatism, traditionalism, and spirituality of Irish culture.
Private vs. Public
Going along with the theme of missed opportunities and that which goes unsaid is the theme of public versus private life. This is presented most starkly in the theatrical representation of Gar's Public and Private selves, played by two separate actors who present distinct perspectives and consciousnesses onstage. It goes to show the audience that everyone—no matter the appearance they present to the world—can have a wildly different aspect to themselves on the inside. Public Gar is quiet, seldom outspoken, rarely shows emotion, and seems to follow authority. On the other hand, Private Gar is witty and crude, especially towards his father who he harshly criticizes throughout the play. He often gets carried away with emotion and feeling. It would be hard to imagine Public Gar acting like this, which illustrates this theme perfectly: that often, what we see on the surface of someone's behavior does not reflect who they are or what they feel on the inside. This theme is further reflected in the way that Gar's father cannot express his love for Gar, as well as his friends' misrepresentation of their own masculinity when socializing, and Madge's inability to be honest about her disappointment with the naming of her grandniece.
Heartbreak
Another theme in the play is heartbreak. Not only is Gar heartbroken by the fact that his father is completely unable to express love, but he is still nursing a broken heart from the loss of his beloved, Kate, who married another man when Gar was unable to muster the courage to propose. Gar thinks of Kate wistfully and remembers the plans they made as a couple, to marry and have many children, but these are all painful memories because they have been dashed. Furthermore, this heartbreak was preventable, as we see in the flashback, which makes it all the more excruciating. While Kate's father extolled the virtues of Gar's rival for Kate's heart, he also invited Gar to ask for his permission for Kate's hand and implied that he would easily grant it. Thus, the dissolution of Gar and Kate's relationship is a tragic and prominent theme in the play.
Immigration
The play takes place on the night before Gar's departure for Philadelphia, where he plans to make a new life working at a hotel as an American citizen. A great deal of the play concerns the tension between Gar's Irish upbringing and his desire for a completely new life. While he has a deep identification with his homeland, with friends, a job, and a community, he is also eager to leave it all behind and start afresh. Emigration is portrayed as an emotionally complex decision, one that is as much about rejection of the old as it is an embrace of the new. While it is never overtly stated, it is implied that Gar's decision to go to America might have something to do with his family and friends' inability to connect with him emotionally on the eve of his departure. In a way, his decision is interpreted as a rejection of the community that raised him. This idea is brought to a devastating climax when Gar has an angry outburst during Kate's visit. He rails against Ballybeg and suggests that he does not want to see it ever again, that he hates it there. Thus we see the complex emotions that accompany immigration, the ways that it is not simply a straightforward journey towards new opportunities, but an often ambivalent and devastating abandonment.
Family
Gar's problems, his frustrations and disappointments, primarily stem from familial problems. In the course of the play, we learn that he lost his mother very early in his life and has no memory of her. As such, her identity is a near-constant topic of thought for him, a continual motif that appears in his mind whenever it is idle. Additionally, he was left behind with a father who can barely express a friendly thought let alone a loving one, and Gar deeply resents his father's inability to express himself or show him affection. In order to get away, Gar has decided to stay with his family members, Aunt Lizzy and Uncle Canon, in America, who are desperate to have a young person around, since they themselves are starved for familial connections. However, Gar is conflicted about whether or not his desire to live with them is a straightforward one or an expression of a deeper need for a maternal figure. Familial lack and deprivation thus make up a great deal of the thematic substance of the play.
Doubt
Gar is crippled by doubt nearly constantly in Philadelphia, Here I Come! His sense of disconnection from his father has to do with his doubt that his father even cares about or loves him. His relationship to his friends is marred by his doubt that they care about him. Then, perhaps most heartbreakingly, his intention to marry and make a family with Kate is completely thrown off course by his doubt, his fear that he is not good enough for her and her father's expectations.
The doubt that Gar struggles with the most is his doubt about whether or not he ought to emigrate to America. He spends most of the play confidently planning his trip, but still cycles through nostalgia and ambivalence all the while. The ending, when he is about to leave, is clouded by Gar's doubt, his inability to tell whether he has made the right decision. "Why do you have to leave?" Private Gar asks Public Gar, to which Public replies, "I don't know." The play ends without resolving this uncertainty.
