I would definitely say wait if you're considering a national school. I spend some of my work time in national primary schools and have done some analysis of the national mathematics and reading tests. It is noticeable how often the kids with the lower scores are younger on average than those with the higher scores.
If you're considering a private school your child stands a better chance insofar as it is easier to repeat a year in a private school, however national schools are allocated teachers on the basis of pupils enrolled and therefore they are reluctant to hold a child back. If a child starts Junior Infants and are not able to cope and need to repeat the year, it means that the next year of Senior Infants has one less, and also there is one less place to offer in the incoming Junior Infants class.
Something else to consider is that it's not just about how intelligent your child is, the motor skills are very important, and it's hard to watch a child who is much younger than most of their classmates trying to deal with what should be a simple task like cutting out a shape on card. They end up way underperforming on much of the artwork which is a large part of the Junior Infant year.
By the way, the child your daughter is being compared against may not necessarily be performing at the national average. I wouldn't advise reading too much into a comparison against one child.
Lastly, although it seems a long way away now, when your child reaches 6th year, her friends will be having 18th birthday parties, often in places where alcohol is served, and your daughter will be far and away too young to go to these parties, which may well leave her feeling resentful.
Provided you keep her well occupied in the meantime, waiting another year will, I believe, be far preferable than starting her off in a situation where she is finding school difficult from the get-go.