The sentence you ask about continues Heaney's humble statement of thanks in his acceptance speech. It continues the subject of poetry as the great source of meaning and truth. Rather than credit himself or his own gifts, Heaney graciously credits the nature of poetry itself for the literature that has garnered him the Nobel Prize. He uses a metaphor of going on a journey with language and with poetry, a journey that turned into a spacewalk, to describe his ascendance in poetry.
The wartime ... was pre-reflective time for me. Pre-literate too. Pre-historical in its way.
Earlier, his journey began with listening earnestly to the family radio. From the radio, he learned a love of language. He suggests that from the love of language grew his love of poetry. This love of poetry--developed from love of language learned from the voices on the radio--suggests that, since his love began in his "pre-historical" period of young childhood, the credit for his greatness--for the Nobel is given to writers with recognized greatness--goes directly to poetry itself rather than to the man, Heaney himself. He suggests that language, specifically the language of poetry, made him great rather than that he made language and poetry great.
The meaning of the quotation must be taken in its full context to understand it:
"... I had already begun a journey into the wideness of the world beyond [my thatched home in Ireland]. This in turn became a journey into the wideness of language, a journey where each point of arrival -- whether in one's poetry or one's life turned out to be a stepping stone rather than a destination, and it is that journey which has brought me now to this honoured spot [at the Nobel platform]. And yet the platform here feels more like a space station than a stepping stone, so that is why, for once in my life, I am permitting myself the luxury of walking on air.
"I credit poetry for making this space-walk [at the Nobel ceremony] possible. I credit it immediately because of a line I wrote fairly recently .... But I credit it ultimately because poetry can make an order [of things] as true to the impact of external reality and as sensitive to the inner laws of the poet's being as the ripples [of water] .... I credit it [poetry] because credit is due to it, in our time and in all time, for its truth to life, in every sense of that phrase."
In context, it's easier to see that what Heaney means is [in paraphrase]: I give the credit for my Nobel honor to poetry alone because poetry deserves the credit. It deserves the credit in our present time and in all of time both past and present. I give credit to poetry because of its ability to capture the truth of life, of life's every moment. I give credit to poetry because of its truth to life in every sense of meaning that the phrase "truth to life" can encompass: poetry's truth to life's pain, truth to life's joy, truth to life's struggles and injustice, truth to life's heroism and virtue, truth to life's loves and even truth to life's hates.
[It is interesting and touching to listen to Heaney's actual Stockholm speech while reading along from the text of his speech. You can hear his nervousness and the genuine humility with which he addresses the assemblage at the Nobel ceremony.]
Seamus Heaney writes from the heart. His experiences inspire him to write plays and create poetry and, in 1995, he won the Nobel Prize for Literature. Brought up as a Catholic in a turbulent Northern Ireland, but on the border with Eire, Southern Ireland, he experienced a protected childhood. As he says in his Nobel speech (Nobel speech 1995) he lived "in the doze of hibernation" and only much later would the impact of childhood experiences become evident in his work, although he became more interested in detective stories than news stories about the Second World War. It was this exposure, first to BBC radio, and later to a combination of stations including Radio Eireann, that started Heaney on "a journey into the wideness of the world beyond." Heaney points out that each achievement, each part of a "journey," is more of a "stepping-stone" to realization. In the speech, Heaney muses on the unlikely opportunity for him even to visit a city such as Stockholm, never mind receive such a prestigious award there.
Heaney equates the winning of the Nobel Prize with a state of "walking on air" because of the fact that an average Catholic boy, with an average Irish childhood, is receiving the award. Just as most people do not even conceive of taking a trip to the moon or into "space," so too could he never have conceived of this accolade. From this realization, Heaney recognizes that poetry is behind his achievements because it made "this space-walk possible." It is because of poetry's ability to release a message and explain a thought, even one from fifty years ago, that he has been able to reach such heights of success.
When Heaney says "Because credit is due to it..." he is acknowledging poetry's contribution to expression and stating that others must also recognize its value. He continues with "...in our time and in all time..." which means that poetry is relevant now and always has been and always will be. When he says "for its truth to life..." he is confirming that poetry has the capacity to reveal the truth and that it is sincere. When he goes on to say "in every sense of that phrase," he is referring to the phrase "truth to life," and suggesting that this could have different interpretations and people must not overlook any of them in considering "every sense," be it the truth people see, what they hear, the ways in which they are affected by circumstances, their experiences, or their intuition.