Religious beliefs are strongly felt and key to people’s every day lives. How could it be that this shift was so large even among the general population?
And of course why was this change so large in England and Scotland but not in Ireland? If the argument is one of brute force, the monarchy was never shy of using that across the Irish Sea.
Like all historical events Martin Luther's theses and the Protestant Reformation have a multitude of factors that are due to broader economic and social factors. It's not like Martin Luther appeared out of thin air - reality is that for hundreds of years prior there were movements advocating reform to the Catholic church: Waldensians, Hussites, John Wyclyff and the Lollards, etc. When we these movements together with the social/economic factors (raising middle class, invention of printing press) the change seems less sudden and more inevitable.
Here are some links to questions about previous Catholic reform movements in medieval Europe:
First of all, 100 years is a long time. It is unlikely that you or I or anyone who is viewing this comment will live to be 100 years old. Especially for premodern history, the scale of time seems to get distorted such that 100 years for a shift in attitudes seems short.
England's conversion to Protestantism started fairly early due to the high mobility of the English gentry, ambassadorial courts and merchant companies that traveled between London and Antwerp, and from there to Cologne, Frankfurt, Paris, etc. Notable early English Protestant reformers such as Thomas Cranmer were involved in Henry VIII's "Great Matter" of divorcing Catherine of Aragon, and many of their arguments suggested appealing to decisions by the heads of continental universities, which presents us with an English intellectual milieu that was cosmopolitan in nature.
The Church itself would not become "Protestant" in most senses of the word until Henry VIII died and his son, Edward, a minor, was in charge of the state. Cranmer took over as the head of religious matters, while the political side passed between ministers of the Privy Council, who were, at least after 1551, sympathetic to the Protestant cause.
While Edward only reigned for 6 years, so did his Catholic sister Mary. The most acrimonious division between Protestantism and Catholicism ironically came under Elizabeth I, who was far more pragmatic about religion provided that her subjects supported her reign. The reason for this hostility was due to the fact that Elizabeth had remained unmarried, and thus was unable to produce an heir. The future of the throne would remain uncertain, and Philip II of Spain, who had married Mary Tudor, had a decent claim not only to the English throne if Elizabeth died, but also offered his own hand in marriage. Although she's generally treated as a religiously tolerant queen by historians, over the course of her reign, Elizabeth ordered the execution of thousands of Catholics within England due to, what she felt was, a lack of loyalty to the Church of England.
In England's case, the Protestant-Catholic divide was almost entirely related to state power, sovereignty and the authority of their monarchs. Catholicism as a practiced religion survived in England (nurtured by foreign missionaries) well into the next century, even as the monarchy under James I became a more nakedly Protestant in sympathies. James himself was targeted by Catholic anti-Tudor monarchists due to the fact that, as the co-regent King of Scotland, he had been influenced by a particularly alacritous form of Calvinism as espoused by ministers like John Knox. James was a Protestant monarch that like the vestiges of power that Catholicism gave in terms of the episcopacy and hierarchy, and the Church of England has survived in this half-and-half nature to this day.
As for Ireland, although the English crown had pretensions to ruling Ireland since Henry II, in reality, they had very little influence until 1609. The Tudors beginning with Henry VIII and his minister Thomas Cromwell had tried different tactics of appointing sympathetic bishops and undertakers to colonize and form "plantations" in Ireland, but these colonies were prone to rebellions and setbacks. It wouldn't be until English-led armies in Ireland defeated the rebellious Earls of Tyrone and Tyrconnel that the Ulster Plantation was able to be constructed. The Ulster Plantation, viewed by many Republican Irish as one of the greatest colonial atrocities perpetrated by the British Empire, lasted in its initial form until the Wars of the Three Kingdoms in the mid-seventeenth century. The territories encompassed roughly correspond to modern day Northern Ireland, which is often characterized by its Protestant and pro-English (relatively speaking) attitudes.