Adj with or without -al

[queenmaabd]

Ok, I should know this, but I don't, when are adjectives ending in -al used in favor of those that don't?
I'll give an example.
He's very ironical.
He's an ironic person.
Does it have to do with having a noun behind it?

THANKS for all your wonderful help.

[Mister Micawber]

Some pairs are synonymous (ironic, ironical); some pairs offer different meanings (historic, historical); some are a result of ignorance (moronic, moronical). The reasons are primarily historical, so you'll need to learn them on a case-by-case basis from the dictionary.

[queenmaabd]

Some pairs are synonymous

some pairs offer different meanings (historic, historical)

OK you say, some are synonims and others aren't, but I've looked as many as I can think of on dictionary.com, and they all show up as synonims (true, moronical doesn't exist). But still, in all definitions, one shows up as a synonim of the other.
ironical= ironic
Hysterical=hysteric
psychological=psychologic
even
spheric=spherical

So I still feel there must be something missing....

[Tdol]

OK you say, some are synonims and others aren't, but I've looked as many as I can think of on dictionary.com, and they all show up as synonims (true, moronical doesn't exist). But still, in all definitions, one shows up as a synonim of the other.
ironical= ironic
Hysterical=hysteric
psychological=psychologic
even
spheric=spherical

So I still feel there must be something missing....

There may well be, but it escapes me.

[Mister Micawber]

In the dictionary, you will find separate entries for 'historic' and 'historical', 'classic' and 'classical', and other adjective pairs that differ in meaning; otherwise, it is just a fickleness of language history that both forms have been considered acceptable at some time.