The speaker of "Fern Hill" reminisces about days he spent on a farm when he was young. He's older now, although it's not clear exactly how old, and spends the whole poem talking about himself when he was younger.
Normally, we would never assume that the speaker in a poem is the author. But we know Dylan Thomas spent his summers at his Aunt's farm in Carmarthenshire, Wales when he was young. And we know that farm was called Fern Hill. So, does that mean the "I" in the poem is Dylan Thomas?
Not necessarily. The speaker may be similar to Thomas and the poem may include some autobiographical details from Thomas' life, but that doesn't mean that they are the same person. In any event, all we really know about the speaker is that he loved being young and maintains that childlike wonder throughout most of the poem.
Also, if we're being honest, this guy's tone can be hard to pin down sometimes. No doubt, earlier on in the poem, the speaker gets caught up in remembering the landscape of his youth. Notice how he comes right out and says, "I was young and easy" (line 1), and "I was green and carefree" (line 10) and "it was lovely" (line 19)? The speaker is being straightforward about how great life was back in the day.
But later in the poem, the mood shifts, while the speaker still manages to keep using the same diction and imagery from earlier in the poem. He repeats that he was young and carefree and "green," but by the last stanza, those words have taken on a different meaning than in the beginning. So our speaker seems to be straightforward, but his memories of youth swerve from joy to sadness.