How to explain irrational numbers to laymen?
Rational numbers are values the can be written as a ratio (fraction) of two whole numbers. Irrationals are those values that can not.
End of story.
The first trick is to realize that there are values that can't be (Pythagoras didn't want to believe it). But if you toss a dart at something a mile wide what is the likelihood that it will hit a value that is exactly a ratio of two whole numbers. If one puts it that way it seems slim.
But the second trick is to wonder, if the value isn't a ratio of whole numbers, then what is it and how can we express it? The hardest thing for novices to get is that question doesn't have an answer. We can't express them.
But fractions can be made to be at least as small and smaller than anything we like. That means although we can not express all values, we can express a rational fraction that can be at least and closer to the value as we'd like.
Now decimals are just fractions. $0.1=\frac 1 {10} $ and $0.3769=\frac {3769}{10000} $. We can get as close to expressing all values by taking longer and longer decimals. But we can never actually express an irrational with decimals. Instead ever Irrational has an infinite string of decimals that get close to it to any possible degree.
And that is what we mean be an irrational number being a series of rational fractions. For every irrational number we can find a series (not just one; many series- the decimal expression is one of them) of rational numbers getting closer and closer together and honing in on the irrational and not honing in on anything else. The series is infinite and if we were immortal gods existing out of time and space we could see the entire infinite series at once and see it converges precisely to the irrational numbers. But we are not immortal gods outside of space so we can not. But we know it does.
Let's start by pinning down the definition of "(ir)rational" precisely, since it's sometimes presented unclearly. A rational number is a ratio of integers: $a$ is rational if there are integers $x, y$ such that $a={x\over y}$. Every number can be written as a fraction, it's the "of integers" part which is crucial. Certainly every number with a terminating (= finitely long) decimal expansion is rational: to see why, think about the (completely arbitrary) example $$254.1345987={2541345987\over 10000000}.$$ (Note how much clearer this is than "a rational number is a fraction"!)
Moving on to the other part of your question: just because a number is a "limit" (which can be made precise!) of rational numbers doesn't mean that it itself is a rational number. In fact every number is a limit of rational numbers (that is, the rationals are dense in the reals) in a very simple way: just consider increasingly long decimal representations! E.g. $\pi$ is the limit of the sequence $$3, {31\over 10}, {314\over 100}, {3141\over 10000},...$$
In general, just because $a$ is a limit of things with property $P$ doesn't mean that $a$ itself has property $P$.
EDIT: At this point it might help to see an explicit example of a "naive limit argument" breaking down. Let's think about terminating decimal expansions. It's easy to show that a number $a$ has a terminating decimal expansion iff $a$ can be written as $p\over q$ where $p, q$ are integers and $q$ is a power of $10$. For example, ${3\over 50}$ fits the bill since ${3\over 50}={6\over 100}$ and $100$ is a power of $10$.
Now think about the number $1\over 3$. This doesn't have a terminating decimal expansion - its (unique) decimal expansion is $0.33333...$. However, we can write it as $${1\over 3}=0.3+0.03+0.003+0.0003+...$$ and $0.3, 0.03, 0.003, 0.0003, ...$ all have terminating decimal expansions. Alternatively, we could write ${1\over 3}$ as the limit (again, in a rigorous sense which I won't define here) of the sequence $$0.3, 0.33, 0.333, 0.3333, ...$$ and each term of this sequence has a terminating decimal expansion. So we can see that even though ${1\over 3}$ can be "approached by" or "built from" numbers with terminating decimal expansions, it itself does not have a terminating decimal expansion.
(Yet another example: $0$ is a limit of positive numbers, but is not positive. And so on.)
Note that none of this addresses the question of how we know that there are irrational numbers; that's a separate problem. There are some numbers which we can rigorously prove are irrational, and more abstractly there is a technique in set theory which shows that most numbers are irrational (and most numbers are transcendental, and ...) in a precise sense. But that's a separate issue.
Any finite sum of rational numbers is rational. Even if it is a sum of millions or quadrillions of rational numbers. I think the step which is hardest to comprehend is understanding the limit that finally leads from only rational numbers to the irrational number.
However, what does it even mean to add infinitely many rational numbers? This is not clear a priori and needs a definition. Let's see how to motivate one.
Some rational sums seem to tend to another rational:
$$1+\frac12+\frac14+\frac18+\cdots.$$
the partial sums $1$ and $1+1/2=1.5$ and $1+1/2+1/4=1.75$ and so on come closer to $2$ from below but never exceed it. It was therefore defined that this sequence converges to the number $2$. This is merely a definition, but motivated by observation.
Whenever we see a sequence of numbers coming closer and closer to a rational number $x$ (precisely: arbitrary close, and staying there), then this number $x$ is called the limit of the sequence. This again is just a definition.
Having these definitions in place, one then discovers, that there are sequences which have strange properties:
- the sequence behaves like a convergent sequences of rational numbers in many aspects, e.g. it is making shorter and shorter steps over time, etc. (see Cauchy sequence)
- there seems to be no rational number which this sequences comes closer and closer to.
So we have two options. We either accept that there are strange "convergent" sequences that do not converge against anything, OR we invent new numbers that fill these gaps. These numbers are the irrational numbers. E.g.
$$3+0.1+0.04+0.001+0.0005+0.00009+\cdots =: \pi.$$
So I recommend thinking about it the other way around. There are no strange rational sequences/sums that suddenly are no longer rational, but the irrational number was placed precisely there were it is in order to give this sequence a limit in the first place. The surprising observation therefore is that without this irrational number in place, there is nothing this sequence/series will eventually converge to. Maybe accepting that some specific infinite sums do not converge at all (while also not oscillating or diverging to $\pm\infty$) is more comprehendable.