The Catholic Haydock Commentary says this about Mt. 5:6:
Ver. 6. Hunger and thirst; i.e. spiritually, with an earnest desire of being just and holy. But others again understand such as endure with patience the hardships of hunger and thirst. (Witham) --- Rupertus understands those to whom justice is denied, such as poor widows and orphans. Maldonatus those who from poverty really suffer hunger and thirst, because justice is not done them. (Menochius) --- They shall be filled with every kind of good in their heavenly country. I shall be filled when thy glory shall appear. (Psalm xvi.)
St. Thomas Aquinas's collection of the snippets from Church Fathers' Scriptural commentary, Catena Aurea (Golden Chain) c. 5 l. 4, says this about Mt. 5:6:
St. Ambrose: As soon as I have wept for my sins, I begin to hunger and thirst after righteousness. He who is afficted with any sort
disease, hath ho hunger.
St. Jerome: It is not enough that we desire righteousness, unless we also suffer hunger for it, by which expression we may understand
that we are never righteous enough, but always hunger after works of
Pseudo-Chrysostom: All good which men do not from love of the good itself is unpleasing before God. He hungers after righteousness who
desires to walk according to the righteousness of God; he thirsts
after righteousness who desires to get the knowledge thereof.
St. Chrysostom: He may mean either general righteousness, or that particular virtue which is the opposite of covetousness. As He was
going on to speak of mercy, He shews before hand of what kind our
mercy should be, that it should not be of the gains of plunder or
covetousness, hence He ascribes to righteousness that which
is peculiar to avarice, namely, to hunger and thirst.
St. Hilary: The blessedness which He appropriates to those who hunger and thirst after righteousness shews that the deep longing of
the saints for the doctrine of God shall receive perfect replenishment
in heaven; then "they shall be filled."
Pseudo-Chrysostom: Such is the bounty of a rewarding God, that His gifts are greater than the desires of the saints.
St. Augustine: Or He speaks
of food with which they shall be filled at this present; to wit, that
food of which the Lord spake, "My food is to do the will of my
Father," that is, righteousness, and that water of which whoever
drinks it shall be in him "a well of water springing up to life
St. Chrys.: Or, this is again a promise of a temporal reward; for as covetousness is thought to make many rich, He affirms on the
contrary that righteousness rather makes rich, for He who loves
righteousness possesses all things in safety.