The poem
Sing of the O'Rahilly, Do not deny his right; Sing a 'the' before his name; Allow that he, despite All those learned historians, Established it for good; He wrote out that word himself, He christened himself with blood. How goes the weather?
Sing of the O'Rahilly That had such little sense He told Pearse and Connolly He'd gone to great expense Keeping all the Kerry men Out of that crazy fight; That he might be there himself Had travelled half the night. How goes the weather?
'Am I such a craven that I should not get the word But for what some travelling man Had heard I had not heard?' Then on pearse and Connolly He fixed a bitter look: 'Because I helped to wind the clock I come to hear it strike.' How goes the weather?
What remains to sing about But of the death he met Stretched under a doorway Somewhere off Henry Street; They that found him found upon The door above his head 'Here died the O'Rahilly. R.I.P.' writ in blood. How goes the weather.?

In the poem Yeats,an admirer, acknowledges 'the' O'Rahilly. He depicts him as being both honourable and brave. It seems O'Rahilly, who considered an uprising would fail, felt compelled to pay with his blood the part he played
If you want to understand the poem "The O'Rahilly", you need to study Yeats the poet, "the" O'Rahilly the person, the Easter Rising, and Irish history in general. I think that it is important to understand that Yeats' "historical" poems were liable to alter the history. Consider, for example, "Cuchulain's Fight with the Sea". Emer in this poem is Cuchulain's jilted wife, who sends their son to avenge the wrong;…
Read the whole commentIf you want to understand the poem "The O'Rahilly", you need to study Yeats the poet, "the" O'Rahilly the person, the Easter Rising, and Irish history in general.
I think that it is important to understand that Yeats' "historical" poems were liable to alter the history.
Consider, for example, "Cuchulain's Fight with the Sea". Emer in this poem is Cuchulain's jilted wife, who sends their son to avenge the wrong; Cuchulain then kills his own son. But in the mythology, it is not Emer who plays this role.
There are other examples.
So we cannot assume that the historical story of the O'Rahilly is correctly presnted in the poem. Yeats may have changed some aspects.
Nevertheless the general story told in the poem appears to be almost exactly correct.
"The" O'Rahilly expected that the Easter Rising would fail. Perhaps all the leaders of the Easter Rising were of this opinion. So what would be the point of having all the Irish nationalist forces participate, to later be imprisoned, executed, what have you? The O'Rahilly made sure that only the Dublin forces participated. That turned out to be enough of a blood sacrifice, but it left many alive to continue the struggle.
However, the O'Rahilly felt that he himself had to offer the sacrifice personally. Otherwise he would be a traitor to the cause or a coward.