How does Yeats's "On a Political Prisoner" relate to Irish Nationalism?

Yeats "On a Political Prisoner" was published in 1920, but written in between January 10 and January 29, 1919, a volatile period in which the Irish nationalist party won a massive electoral victory and declared Irish independence from Britain. This led to the Anglo-Irish war in which much of Ireland achieved independence from England by the end of 1921. When Yeats wrote the poem, he did not know the outcome of the unrest, but its backdrop as a turbulent period in Irish history in which the Irish capitalized on the exhaustion of the British in the aftermath of World War I informs the verses.
Although Yeats, especially in the 1890s, had been a supporter of Home Rule (Irish independence from Britain) by the time he wrote this poem he had become disillusioned with the direction of Irish nationalist politics. This is reflected in the poem.
As the title suggests, it is centrally about politics, though its themes are couched in universal terms. It questions the mindset of the central character, a woman who has been imprisoned for her politics. The person addressed by the speaker is often taken to be Maud Gonne, a woman he was long in love with although she eventually married someone else. The speaker questions her in tones critical to her politics, implying they have made her thinking rigid and hateful:

her mind
Became a bitter, an abstract thing


He contrasts this to her youthful character, which he describes as closer to nature and the beautiful countryside. In more universal terms, the poem criticizes what politics—including Irish nationalism—can do to a person.

In "On a Political Prisoner" Yeats laments how one of his closest friends, Maude Gonne, has turned from being a respectable member of the Anglo-Irish gentry to a radical Irish nationalist. Yeats is supportive of the cause to which his friend has devoted her whole adult life but still regrets how her devotion to that cause has turned her mind into a "bitter...abstract thing." In other words, Maude's mind has become steeped in a rigid, inflexible ideology which Yeats finds repellent.
Yeats gives the impression that the active political life that Maude has chosen is somehow not lady-like, especially not for someone of her impeccable social pedigree. He'd much rather see her as she used to be all those years ago, riding to the hunt under Ben Bulben amidst the extraordinary natural beauty of the Sligo countryside.
But as he ruefully acknowledges, patience has never been one of Maude's virtues. From her childhood on, she's been a fearless free spirit, itching to leave the "nest" which perched upon the "lofty rock" of her safe, secure aristocratic upbringing for the turbulent world of revolutionary politics beneath.
Still, Yeats is big enough to acknowledge that such single-minded devotion to Irish nationalism has given Maude a certain inner peace and calm, and for that at least he's grateful.
https://www.poetryfoundation.org/poems/57316/on-a-political-prisoner

“On a Political Prisoner” describes a woman who has learned patience in prison to the extent that birds will now alight and perch on her fingers, such is her stillness and calm. The contrast with her former impatience for political change reflects the long years she has spent espousing and being punished for espousing what must often have seemed the lost cause of Irish Nationalism.
Yeats’s attitude to the Irish Nationalist women he knew was always ambivalent. He admired their commitment and sympathized to some extent but often felt they were wasting lives that should have been spent reveling in youth, beauty, poetry, and aristocracy rather than wedded to radical politics.
This sense of waste and frustration is evident in the second stanza, where the mind of the subject, however calm, is “a bitter, an abstract thing.” Devotion to a popular cause has made her:

Blind and leader of the blindDrinking the foul ditch where they lie

The third stanza recalls the subject long ago, before she devoted herself to the cause—a natural beauty surrounded by the beauty of nature. The very absence of Irish Nationalism or any form of politics from this stanza gives it a sunlit atmosphere of innocence which lasts until the desperate expression of longing in the final stanza, which could be taken as a cry for both personal and political freedom.

This poem concerns a female Irish nationalist who has been thrown into prison for her part in protesting the rule of Ireland from mainland Britain. It is generally believed that the subject of the poem may be Maud Gonne, an Irish actress and female nationalist with whom Yeats was, for many years, infatuated, and whose husband was executed for his participation in the 1916 Easter Rising. Gonne was very passionate about the cause of Irish nationalism and was active in the cause for many years.
Whether or not the poem is specifically about Gonne, however, it certainly expresses a romantic sympathy for those women who, like Gonne, were passionate enough about Irish nationalism to be imprisoned for their actions in support of it. The woman's mind has become "bitter, an abstract thing" as a result of her dedication to this cause, but Yeats makes clear that she has not become hard or unwomanly—on the contrary, he shows the prisoner gently touching the "lone wing" of a grey gull who flew to her cell, as if sensing her inherent goodness. Like the bird, the poem seems to suggest, the woman longs to be free, but moreover she longs for something beyond this, as the bird "cried out for the hollows of the sea." The freedom the woman knew before her mind became taken up by the cause is insufficient; she now knows that it was not the independence to which she is now dedicated.
https://ireland-calling.com/wb-yeats-maud-gonne/