Irrational numbers are limits of sequences of rational numbers.
Multiplying two real numbers is simply taking the limit of a sequence of multiplications between rational numbers that converge to the two real ones.
That's a pretty far departure from the original "multiplication is just repeated addition". Regardless, I don't think any student would find it helpful to hear "Multiplying two real numbers is simply taking the limit of a sequence of multiplications between rational numbers that converge to the two real ones". In my country irrational numbers are introduced two or three years before limits so you couldn't teach it in schools effectively either.
Multiplication outside of positive integers is not "repeated addition".
It took us thousands of years to properly define real numbers. High school students can live without a perfect explanation, or we can just teach limits before college since they are the fundamental concept if calculus.
Multiplication is repeated additions is the informal way of stating the distributive property of multiplication and addition.
Probably you were taught how to multiply irrational by the property of powers (a^b * c^b = (a*c)^b etc.).
You were not taught a grand unifying theory of multiplication, you were taught how to manipulate operations to turn them into more useful operations.
Teaching these laws also prepares you for when a and b are just symbolic reals with no structure and those laws are the only thing you can use to manipulate them.
You don't need a rigorous notion of limits to informally notice that irrationals have arbitrarily close rational approximations, e.g. by adding successive digits.
It's a kind of generalization of repeated addition. Once you've been taught about π, you know that 2π lies between 23 and 24, without being told about limits. It may not be rigorous, but it's a good start.
You can't teach "the truth" (whatever you hold that to be). It would set back education instead of advancing it. In this case too, perfect is the enemy of good.
