I grew up in England but we all ways had "getting up to that mallarkey" meaning being silly, I never heard it used to mean talking rubbish

They also say in Dublin - "Acting the maggot" - with the same meaning.

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It is recommended that you always wait for three to agree on a translation.
I speak Connemara Irish, and my input will often reflect that.
I will do an mp3 file on request for short translations.
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I have lived in Dublin all my life and never heard malark(e)y used in either of those ways. I can't imagine anyone I know using the word, except in a mildly humourous way (or perhaps to stress real disapproval about someone's 'carry on') - i.e. using the word at all accents the sentence because of how unnatural the word is to us. In my experience, it would be used in reference to actions rather than words.

"It's time we had a bit of a tête-à-tête about yer wan's malarkey". To me, 'malarkey' would be as 'foreign' as 'tête-à-tête', and used in the knowledge of it being so. While both phrases could be used, they wouldn't be used without the speech sounding somewhat 'affected' (for lack of a better word).

Yeah, 'acting the maggot', would be 'acting the eejit' or getting up to some (harmless) mischief. I had no idea it was just a Dublin thing.

Personally speaking I've only heard it on television. It may be more widespread than Dublin.

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___________________________________________________________

It is recommended that you always wait for three to agree on a translation.
I speak Connemara Irish, and my input will often reflect that.
I will do an mp3 file on request for short translations.
___________________________________________________________

It seems to me that a lot of slang in any area only lasts for one generation. There is a site called "Overheard in Dublin" that I look at from time to time. Much of the Dublin slang on that site is all new to me
and of a very dismissive type. Generally Dublin slang is of a very witty type.