A public expectation, it has to be said, not of poetry as such but of political positions variously approvable by mutually disapproving groups.

Heaney is describing the pressure placed on poets—especially in politically charged contexts—to serve as public spokespersons rather than as makers of poems. The “public expectation” he names is not a demand for poetry’s imaginative or formal achievements, but for declarative stances that can be claimed by one faction and rejected by another. The phrasing suggests a weary precision: he concedes the expectation exists (“it has to be said”) while distancing poetry from partisan utility. Implicitly, he defends poetry’s autonomy and complexity against reductive readings that treat poems as political position papers.