There aren't too many nationalities represented in Tom Jones: the big majority of characters are English. Mr. Fitzpatrick is an exception, being Irish. In a lot of ways, he comes across as an obnoxious English stereotype of an Irishman, with his quick, violent temper and his jealousy over his wife, Harriet Fitzpatrick.
But Mrs. Fitzpatrick does state, against Sophia's clear anti-Irish prejudice, that there are lots of good guys—and good husbands—in Ireland: "There are, among the Irish, men of as much worth and honour as any among the English" (11.7.27). Mrs. Fitzpatrick's main problem is that she married a fool, not that she married an Irishman. Still, in spite of Fielding's apparently open-minded statement about Irishness from the mouth of Mrs. Fitzpatrick, his overall portrayal of Mr. Fitzpatrick as a hotheaded idiot seems biased.
Like many of the other characters in this novel, including Mrs. Honour, Mr. Fitzpatrick sometimes appears less like a character and more like a plot device. So, for example, Mr. Fitzpatrick's jealousy leads him to run into Mrs. Waters's room at the inn at Upton by mistake, surprising Tom in bed with her. Without this accident, Susan the chambermaid might never have told Sophia that Tom and Mrs. Waters are sleeping together at that very inn.
Similarly, Mr. Fitzpatrick arrives in London just in time to find Tom leaving Mrs. Fitzpatrick's residence. Without the duel that follows, Tom would never have been thrown in prison to face his final crises of the novel.
Our sense that this sudden reappearance of Mr. Fitzpatrick's is a (kind of lame) plot device only seems more likely when we remember that Mr. Fitzpatrick disappears from the novel after this fight. Suddenly, following his sword injury from Tom, he's too happy with Mrs. Waters to care about his wife or the guy who injured him in a duel. That seems totally against the jealous, fight-happy character that the novel has given us up until Book 18.