What is the history of Irish Catholic resentment toward Protestantism? In the Pulitzer Prize-winning book, Angela's Ashes, the author depicts several instances of Irish Catholic animosity toward Protestants, especially those from America. What is the background of this rift, and does it persist today?
The history of religious conflict in Ireland is far too complex to tear down in a single answer, but this should provide some clues for further reading. For background what you need to read up on is not actually anything specific to Christianity so much as the overall political history of Ireland. For example Wikipedia says the following in its introduction to the topic:


  The Crown did not attempt to assert full control of the island until
  after Henry VIII's repudiation of papal authority over the Church in
  England and subsequent rebellion of the Earl of Kildare in Ireland
  threatened English hegemony there. Henry proclaimed himself King of
  Ireland and also tried to introduce the English Reformation, which
  failed in Ireland. Attempts to either conquer or assimilate the Irish
  lordships into the Kingdom of Ireland provided the initial impetus for
  a series of Irish military campaigns between 1534 and 1603. This
  period was marked by a Crown policy of plantation, involving the
  arrival of thousands of English and Scottish Protestant settlers, and
  the consequent displacement of the pre-plantation Catholic
  landholders. As the military and political defeat of Gaelic Ireland
  became more pronounced in the early seventeenth century, sectarian
  conflict became a recurrent theme in Irish history.
  
  The 1613 overthrow of the Catholic majority in the Irish Parliament
  was realised principally through the creation of numerous new boroughs
  which were dominated by the new settlers. By the end of the
  seventeenth century, recusant Roman Catholics, as adherents to the old
  religion were now termed, representing some 85% of Ireland's
  population, were then banned from the Irish Parliament. Protestant
  domination of Ireland was confirmed after two periods of war between
  Catholics and Protestants in 1641-52 and 1689-91. Political power
  thereafter rested entirely in the hands of a Protestant Ascendancy
  minority, while Catholics and members of dissenting Protestant
  denominations suffered severe political and economic privations at the
  hands of the Penal Laws.


The context in which religion is mentioned in that article should give you a pretty good clue but let me spell it out. Ireland was torn apart for centuries by an ongoing struggle for domination by various monarchies or other political powers. (e.g. War of the Roses, Hundred Years' War, and others). The various parties involved in the struggle each brought with them their own religious affiliation, and the one-to-one pairing with political positions and religious traditions meant each successive party's struggle for power became strongly associate with that party's religious affiliation. In fact those identities were often conflated.

Does this animosity persist today? Yes, but not universally. Many many Irish individuals from both religious camps have individually worked out their issues. Many many churches and larger groups have waded in to turn the tide on the animosity. Yet still pockets of animosity persist.

The best summary of the problem I know of is found in a ballad called There Were Roses written by Tommy Sands. If you've never heard it, it's worth a listen. There are a thousand covers to suit most musical tastes, but the lyrics are a poignant summary of the problem.


  […]
  
  Now Isaac he was Protestant and Sean was Catholic born
  But it never made a difference, for the friendship it was strong
  And sometimes in the evening when we heard the sound of drums
  We said it won't divide us, we always will be one
  
  […]
  
  Now fear it filled the countryside there was fear in every home
  When late at night a car came prowling round the Ryan Road
  A Catholic would be killed tonight to even up the score
  Oh Christ it's young McDonald they've taken from the door
  
  Isaac was my friend! he cried, he begged them with his tears
  But centuries of hatred have ears that do not hear
  An eye for an eye, it was all that filled their minds
  And another eye for another eye till everyone is blind
  
  […]