If an argument is semantically (truth preservation) valid, it usually will be syntactically (derivation) valid.
Counterexample method
Instead of changing the words in the argument, we can change the situation we’re in.
I.e.
Instead of changing
Most Irish are Catholic, most Catholics are against abortion, so there exists some Irish people that are against abortion
to
Most chess GMs are men,
most men are no good at chess
so there exist chess GMs that are no good at chess.
This, however, doesn’t address the original argument.
Instead, you can switch the situation. For example, through a diagram of the demographic/religious/political situation in Ireland.
An invalid argument form (which is what we can prove with a counterexample) doesn’t necessarily mean every argument instantiating it is invalid.
Logical validity
Logical validity, like deductive validity, is necessary truth-preservation. However, this must happen on purely formal grounds.
“n is a widow, so n is a woman” is a deductively valid argument. However, we need definitions that go beyond formality for this - the idea that “a widow” is necessarily “a woman”. Hence, this is not logically valid.
an argument is logically valid
if and only if
it is necessarily truth-preserving in virtue of the topic-neutral words that occur in its premises and conclusions
every argument that is logically valid, is also deductively valid.