"盛りなる御代..." It's from Genjimonogatari.
My two cadidates are either もる ·さかる, so "mori-naru" ro "sakari-naru".
I can tell the meaning of the word, but I'm not sure which way I should read in this context, and in this specific book.
さかる is translated by jisho.org as to prosper; to flourish, and もる is to serve/to pile up.
The definition you want there is definitely to prosper; to flourish since 御代 is defined as an imperial reign.
Putting it together, 盛り{さかり}なる御代{みよ} is a prosperous imperial reign.