Maybe they wiped up after themselves.
Maybe they carried off the women and children they’d fathered.
Maybe their fairly small numbers resulted in the usual power of two reduction erasing most traces.
Maybe they carried off the women,only the good looking ones.
Perhaps their were rules, something akin to those against miscegenation, that were more scrupulously obeyed than we moderns believe.
My impression is the Vikings made much less of an impact on Ireland than Britain. Less land occupied for a much shorter time. I suppose Britain was much more lucrative looting.