The poem "Easter 1916" by William Butler Yeats was written as a reaction to the Easter Rebellion of 1916, also known as the Easter Rising. We'll provide some background to the event. We'll also explain why Yeats wrote the poem and what the lines you quoted mean. Then you can compose your own thoughts on the subject.
In the Easter Rebellion, approximately 1,000 Irish Republicans started an insurrection in April of 1916. Their objective was complete secession from Great Britain and the establishment of an independent Ireland. The British crushed the rebellion within a week and executed many of the perpetrators by firing squad. The brutality gave rise to a widespread underground nationalist movement in Ireland.
In the poem, Yeats focuses on four participants who were killed in the rebellion. In the first stanza, he writes that he used to pass them in the streets before the uprising. In the second stanza, "that woman" refers to Constance Gore-Booth Markievicz, a politician. "This man" was Patrick Pearse, a poet. "His helper and friend" was another poet named Thomas MacDonagh. A man named John McBride was the "drunken, vainglorious lout." Yeats lists all these people by name in the final stanza of the poem. He describes them not as heroes, but as ordinary, imperfect people who died during the uprising.
Because these people who Yeats used to pass in the streets and others like them were killed by the British, they have become martyrs to the cause of rebellion. That's why Yeats says near the end that "now and in time to be, wherever green is worn" (green is symbolic of the Irish nation and people) everything has utterly changed. Yeats considers an independent Ireland to be something beautiful. However, its beauty is terrible. The birth of the Irish nationalist movement has been purchased with the price of violence, bloodshed, and death.