The poet had "met them at close of day" and passed with "polite meaningless words" before "all changed, changed utterly: / A terrible beauty is born."  
The man who "might have won fame in the end, / So sensitive his nature seemed" was Thomas MacDonough, while the man who had seemed "a drunken, vainglorious lout" before resigning his part in the casual comedy was Major John MacBride.  The poet had "met them at close of day" and passed with "polite meaningless words" before "all changed, changed utterly: / A terrible beauty is born."  FTP, identify this poem about a failed Irish nationalist uprising, written in the titular year by William Butler Yeats.
The man who "might have won fame in the end, / So sensitive his nature seemed" was Thomas MacDonough, while the man who had seemed "a drunken, vainglorious lout" before resigning his part in the casual comedy was Major John MacBride.  
"Easter 1916"