Gardai have been told that if their beards resemble "a badly watered lawn in a heatwave" after two weeks' growth, they should "concede graciously and admit defeat."
Hairy-faced cops have been warned to "smarten up" and make a decision if they, or their partners, want them to have beards or not.
The crackdown on claims of grooming rule breaches comes after a change in policy last July, when Commissioner Drew Harris made a decision to let cops to sport facial hair other than moustaches.
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In an email to officers in the Roads Policing Division in Dublin from a senior cop, gardai are reminded of the requirement in the dress code, which is that beards are neatly trimmed and should be between 0.5cm and 2cm in bulk.
It also adds members should grow their beards during annual leave or rest days to avoid an "unshaven" or "stubbly" look.
And the email states: "There are some members who cannot seem to make up their mind whether they want a beard or not, and alternate on a weekly basis from being clean-shaven to having various stages of facial hair.
"This too is unacceptable.
"The decision to allow members to wear beards should not be interpreted as an excuse to break from shaving every now and then.
"In short, members should decide whether they want a beard or not (or whether their partners will allow them to have one or not) and commit to one look or another."
It goes on to say that a beard should not take too long to grow, and the guidelines allow for a maximum of two weeks.
The email added: "If after that, the face still resembles a badly watered lawn in a heatwave, then it is time to concede graciously and admit defeat.
"Beards are not for everyone after all, and as the summer months approach and the days get warmer, there are certain advantages to being clean shaven."
It is the latest warning shot to frontline gardai after those based in Kerry were told last September that they are not in a "Conor McGregor, ZZ Top or Grizzly Adams look-a-like competition."
The major rule change last year came after 20 years of campaigning from the Garda Representative Association. It is also thought to have been influenced by attempts to get more ethnic minorities to join.
The issue was raised as far back as 2001 by Garda whistleblower John Wilson, who was confined to indoor duties for breaching regulations by letting his facial hair grow because of a skin condition.
But even after exemptions for medical reasons were brought in, a motion was passed at the 2017 GRA annual conference calling for all cops to be allowed bare bristles.
But while there was relaxation on facial hair, cops remain banned from wearing jeans or runners even if in plainclothes. Face and neck tattoos also remain prohibited.
Contacted for comment, a garda spokesman said: "A policy was issued in July 2021 on Uniform and Dress Code within An Garda Siochana which includes provisions on beard wearing which all Garda member are expected to follow."
