Dominions of old
The Mariners had great profit and luck, from their strongholds on the continental shore,
Bases that served well, for trade and their spreading of power all along the mortal ways,
Great harbours and ports, shelter for innumerable quantity of ship-craft,
Firm anchor in a world growing unrest and the flame of war, for those were the merriest days of whom used to dwell between the two worlds, neither menaced by pending peril nor completely within the beatitude of never-ending bliss.
The last verse refers to the special condition of Númenor during the dawn of its empire. They had been granted by the Powers a whole new island to reside in, away from the troubles of Middle-earth, closer to the eternal joy of Aman that Men could only gaze at from afar, being them forbidden to pass across the immortal waters. And it is this in-between nature of this realm, placed in more fortunate spaces than the common tribulation of the continent, albeit it being bound to the same ban from the West, that prepared the ground for the infamous rebellion of a one-time glorious lineage of kings.