ALL IN GOOD TIME = glacaimis an gnoithe i ndiaidh a chéile(OK, so gnoithe is gnó in standard Irish.)
BETTER LESS, BUT BETTER = is fearr beagán go maith ná mórán go holc
DON'T GET US INTO TROUBLE = ná tarraing seantithe anuas orainn
the GRIM REAPER = an Rógaire Bás
HAVE-NOTS AND HAVE-LOTS = na daoiní ar an ghannchuid agus na daoiní ar an anchuid
HELP YOURSELF! Tarraing ort!
I'LL LOOK INTO YOUR CASE. Breathnóchaidh mé do chás. (No preposition, a simple transitive verb.)
IN GOOD WORKING ORDER = i ngléas (= electrically or mechanically connected. You remember movies about pre-WW1 pilots asking the mechanic: Contact? and being answered Contact! In Irish, they would say I ngléas? - I ngléas!)
IS POPE CATHOLIC? An dtuigeann an Pápa Laidin?
IT CAME TRUE = tháinig sé isteach, tháinig sé isteach fíor, tháinig an tuar faoin tairngreacht.
It's getting LATE might be quite OK to translate as tá sé ag éirí deireanach, but if you want to sound like the old Gaeltachtian, note teh following idiomatic expressions:
tá sé ag druidim chun deireanais
tá an deireanaí ag teacht
tá an lá mall
tá sé mall sa lá
tá sé ag dul anonn sa lá
tá sé ag teannadh amach sa lá
LET BYGONES BE BYGONES = fág na seanchairteacha
LET SLEEPING DOGS LIE = ná bain fuil as seancholm; is furasta seanaibhleoga a lasadh
OLD NICK = an Sean-Bhuachaill, an Fear Dubh
PERK UP, OLD PAL! Breabhsaigh ort, a mhac!
RULE OUT (i.e. exclude a possibility): De Valera didn't rule out violence as a solution is in Máirtín Ó Cadhain's Irish Níor bhain de Valera foréigean as margadh mar fhuascailt. This duanóg (a gem, when it refers to something someone said or wrote, is duanóg in Irish, if you don't mind) is found in his political manifest, Tone Inné agus Inniu. Anyway, to rule out something is rud a bhaint as margadh, if Máirtín can be trusted.
STILL GOING STRONG! Ar a sheanléim i gcónaí!
TAKE YOUR PICK! Bain do rogha astu!
TAKE YOUR TIME! Tabhair do théarma! Tabhair do sheal!
WATCH YOUR STEP! In the literal sense, i.e. look at where you put down your feet, it is togh do choiscéim!, i.e. choose your step carefully. In the sense of "watch yourself, there might be someone who is out to do you harm" it is seachain thú féin.
WHAM BAM THANK YOU MAM! Ní searc go scaoileadh! (Thanks to Gaelic-L.)
WHEN MEN WERE MEN, WOMEN WERE WOMEN, AND LITTLE HAIRY MONSTERS FROM ALPHA CENTAURI WERE LITTLE HAIRY MONSTERS FROM ALPHA CENTAURI = nuair a bhí an saol filiúnta
WISE UP! Bíodh crothán céille agat! Bíodh an trí splaideog chéille agat! (If it means more like "Grow up!", you can use these ones:) Aosaigh suas! Aosaigh ort!