What does "nubbed" mean? Something is nubbed if it has a nub or nubs. (Just like something is fringed if it has fringes, or something is winged if it has wings.)
The definition of nub is (from Oxford Dictionary Online):
1 The crux or central point of a matter.
2 A small lump or protuberance.
Normally, nubbed would mean having nubs with respect to definition 2. But this is poetry, and it's quite possible that Seamus Heaney is playing with words, so you will have to use your own judgment.
And Gareth Rees has an excellent answer dealing with the rest of your question (although he misses the possible play on words I describe above).
Added in an edit: Some more thoughts about the poem.
"The longship's swimming tongue" says
in the word-hoard, burrow
the coil and gleam
of your furrowed brain.
trust the feel of what nubbed treasure
your hands have known.’
To me, this sounds like the speaker is a dragon (dragons have coils, hoards they burrow in, and are associated with Viking longships). But metaphorically, Heaney is talking about poetry. So literally (from the dragon point of view) "nubbed treasure" means treasure with small protuberances. But metaphorically, it might mean valued poems with "nubs" in sense 1 of the word above.