Small "c."
Denoting universality.
St. Patrick was born *1000* years before the Protestant thing, and even 600 years before the Rome/Constantinoiple split. (Not casting shade on the unfortunate events of the 11th and 16th century here, just noting facts.) That was when catholic was the only Church: one flock, one Shepherd: precisely Catholic and catholic in the most robust sense of the word.
After he escaped from slavery in Ireland, Patricius furthered his education and studied Catholic Christianity in the Lerin Monastery in France.
He later He went to the Monastery in Auxerre where it was decided that a mission should be sent to Ireland
Patrick was made a Bishop by Pope Celestine in the year 432.
So being monastery-trained, Patrick took a great interest in education, and his emphasis on Scriptures planted the seeds for Ireland becoming famed as the "Island of Saints and Scholars." He erected both Catholic monasteries and dioceses in Ireland, and ordained priests.
God bless Protestants one and all, dear brothers and sisters, but face facts: St. Patrick was really quite the catholic Catholic. .