Pelagius' background

Wikipedia gives a compact summary of the life of Pelagius

Pelagius was born about 354. While his exact birthplace is not known, the Encyclopedia of World Biography states that "widespread evidence indicates that he came originally from the British Isles", although a few sources suggest he may have been born in Brittany in modern France. [Paula K. Byers, Encyclopedia of World Biography (1998) p.189 - Pelagius,]

He was a Culdee Monk and wore the moon shaped tonsure of that ascetic Celtic Johannine Christian Order. He became better known c. 380 when he moved to Rome to write and teach about his ascetic practices.There, he wrote a number of his major works:

• De fide Trinitatis libri III ("On Faith in the Trinity: Three Books")
• Eclogarum ex divinis Scripturis liber primus ("Excerpts out of Divine Scriptures: One Book")
• Commentarii in epistolas S. Pauli ("Commentary on the Epistles of Saint Paul")

Unfortunately, most of his work survives only in the quotations of his opponents. Only in the past century have works attributable to Pelagius been identified as such. Pelagius's commentary on Romans is currently available in English, as translated by Theodore De Bruyn (Clarendon Press, 2002), as well as a collection of other writings by Pelagius himself, translated into English by B. R. Rees (The Boydell Press, 1998).

In Rome, Pelagius became concerned about the moral laxity of society. He blamed this laxity on the theology of divine grace preached by Augustine, among others. Around 405, it is said that Pelagius heard a quotation from Augustine's Confessions: "Give me what you command and command what you will". This verse concerned Pelagius because it seemed that Augustine was teaching doctrine contrary to traditional Christian understandings of grace and free will, turning man into a mere automaton.

When Alaric sacked Rome in 410, Pelagius and his close follower Caelestius fled to Carthage where he continued his work and briefly encountered St. Augustine in person. He was subsequently in the Holy Land as late as 418.
wikipedia

Early Culdee monks in Ireland, Scotland?
The summary suggests that the order of ascetic Culdee monks existed in British Isles already on the 4th century AD. Credence to such early presence of Christianity in Ireland and Scotland - when continental Europe beyond Rome was still largely pagan - is given in one of the two authentic letters by the Apostle of Ireland, Saint Patrick (386-640 AD).

The second piece of evidence that comes from Patrick's life is the Letter to Coroticus or Letter to the Soldiers of Coroticus, written after a first remonstrance was received with ridicule and insult. In this, St. Patrick writes an open letter announcing that he has excommunicated Coroticus because he had taken some of St. Patrick's converts into slavery while raiding in Ireland. The letter describes the followers of Coroticus as "fellow citizens of the devils" and "associates of the Scots [of Dalriada and later Argyll] and Apostate Picts". Based largely on an eighth century gloss, Coroticus is taken to be King Ceretic of Alt Clut. It has been suggested that it was the sending of this letter which provoked the trial which Patrick mentions in the Confession. [Todd, James Henthorn (1863), "The Epistle on Coroticus", St. Patrick, Apostle of Ireland, Dublin: Hodges, Smith, &Co. (published 1864), pp. 383–385]
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The language is ecclesiastic but if the Picts were already apostates at the time of St. Patrick writing we may justly suspect that there is at least some earlier history of Christianity in the region that is reflected by these words.