I use a red dot for Bullseye but also shoot iron sights in other
competition disciplines. The Merit iris really helps me maintain
focus on the front sight while not completely losing the target
picture; I imagine it is somewhat of a personal preference that may or
may not do the same thing for others.

Also you should check out the other iris devices from Champion's
Choice, etc. There are some that are a clip-on 'flip up' design [like
the occluders that many bullseye shooters wear over their 'other'
eye;] I could do without the suction cup attachment of the Merit.

Some say that the Merit will help with red dots as well, I have tried
it and can only say it did not hurt, have no basis for yea or nay on
that point.

My vote: try the Merit for iron sight guns, but you will be better off
with a red dot.