Simple RAW for 5e V: The Disadvantage Strikes Back

[E’Tallitnics]

Q89

If a spell requiring concentration is cast into Ring of Spell Storing and then cast from it, does it still require concentration and from whom - ring's owner or original caster

A89: “While wearing this ring, you can cast any spell stored in it. The spell uses the slot level, spell save DC, spell attack bonus, and spellcasting ability of the original caster, but is otherwise treated as if you cast the spell.”

So if the spell requires concentration it still does once cast, and the caster is the one doing the concentrating.