I'm not trying to be pedantic but only a court can issue a 'fine' after a plea of guilty or the defendant being found guilty by the court/jury.
What a Garda issues is a fixed penalty notice, the person getting one can have the matter referred to a court ie choose not to pay it and have a court decide.
If a person is convicted and fined by a judge they have no choice but to pay it or risk imprisonment...there is a difference
Nothing is actually given to the offender "on the spot".
This is one of the reasons so many cases have been struck out in court and, for some reason, hasn't been corrected yet. The notices sent in the post are not registered so it's difficult to prove an offender received their notice.