Everett Ferguson, in Church History, I, 17.I, summarizes Roman Catholic response to this question as follows:
Roman Catholics have defended Honorius's orthodoxy (and so papal infallibility) by various explanations:
(1) he used "one will" in a moral, not physical, sense;
(2) his was a private view not expressed ex cathedra;
(3) the council was wrong in attributing to him the same view as the others condemned, and because of his careless use of language he was condemned along with others;
(4) his name was substituted for another's in falsified acts of the council
As mentioned in Ben's answer, broader discussion that deals with all four of these points, as well as a few others, is available in the Catholic Encyclopedia's article, in the section "Modern controversies on the subject." It also includes the names of prominent advocates of various defenses.
Regarding Ferguson's (1) and (2), the CE says:
The chief advocates of papal infallibility, for instance, such great men as Melchior Canus in the sixteenth century, Thomassinus in the seventeenth, Pietro Ballerini in the eighteenth, Cardinal Perrone in the nineteenth, have been careful to point out that Honorius did not define anything ex cathedra. But they were not content with this amply sufficient defence. [...] Most, if not all, showed themselves anxious to prove that the letters of Honorius were entirely orthodox.
Regarding (3), it continues:
The learned Jesuit Garnier saw clearly, however, that it was not as a Monothelite that Honorius was condemned. He was coupled with Sergius, Pyrrhus, Paul, the Ecthesis, and the Type. It is by no means clear that Sergius, Pyrrhus, and the Ecthesis are to be accounted as Monothelite, since they forbade the mention of "one operation"; it is quite certain that Paul and the Type were anti-Monothelite, for they prohibited "one Will" also. Garnier pointed out that the council condemned Honorius for approving Sergius and for "fomenting" the dogmas of Pyrrhus and Paul.
The CE considers (4) "untenable," writing:
Bellarmine and Baronius followed Pighius in denying that Honorius was condemned at all. Baronius argued that the Acts of the Council were falsified by Theodore, a Patriarch of Constantinople, who had been deposed by the emperor, but was restored at a later date; we are to presume that the council condemned him, but that he substituted "Honorius" for "Theodorus" in the Acts.
Much more detailed analysis of the whole affair can be found in other sections of the CE article, for a better understanding of the intriciacies of Honorius's letters and their place in the debate, and the subsequent condemnation.