Across:
1 Contemporary (MODERN) of Frenchman (Monsieur = M-) with flying (= anagram indicator) drone (drone = -ODERN) (MODERN),
4 Organisation (United Nations = UN-) with old (-O-) pair of writers (pen + ed = -PENED) (UNOPENED) is not exposed (UNOPENED),
9 Pressure (DURESS) to get top upholstery (top “upholstery” = -U-) into Mini (D-RESS) (DURESS),
10 It doesn’t cost anything (FREE) to put it on a horse (REIN) (FREE REIN) with a lack of restraints (FREE REIN),
12 Most of (d) the ocean-liner (‘sand’ without ‘d’ = SAN) in Dublin (MARINO) (SAN MARINO) produced by European republic (SAN MARINO),
13 There’s a short (l) spring (‘well’ without ‘l’ = WEL-) in Donegal (D-L) (DWELL) – just take a moment to think about that (DWELL),
14 Final decision (LAST JUDGMENT) relates to those going up and those going down in the world (LAST JUDGMENT),
18 Massive amount of (ASTRONOMICAL) turf (ASTRO- turf) on (on = -NO-) Rise (= reversal indicator) has a shiny mineral normally found in granite and other rocks (-MICA-) near lake (-L) (ASTRONOMICAL),
21 It’s inevitable without event (it’s “inevitable” without “event” = iiabl = ALIBI) synonymous with cover story (ALIBI),
22 Qualification (AMENDMENT) for American (A-) repair (-MEND-) workers (-MEN-) at the end of employment (at the end of “employment” = -T) (AMENDMENT),
24 Bishop (B-) does someone a service (-ASSISTS) (BASSISTS) with musicians (BASSISTS),
25 It’s said a lot (MANTRA) – no pets in apartments (no ‘pets’ in ‘apartments’ = aartmn = MANTRA),
26 A (A-) Cork town (-BANDON-) with back roads (back “roads” = -S) (ABANDONS) and ditches (ABANDONS),
27 Small (S-) story (TA-LE) about power (-P-) (STAPLE) that binds things together (STAPLE).
Down:
1 Without boasting (MODESTLY), it’s my oldest (my oldest = MODESTLY) puzzle (= anagram indicator),
2 At the top of the dockyard (at the top of the “dockyard” = D-), ship of biblical proportions (-ARK-) is a monster (Loch -NESS monster) (DARKNESS) only seen at night (DARKNESS),
3 Part of mantra starts (part of ‘mantra starts’ = RASTA) with a member of religious movement (RASTA),
5 Zoomed (ran = NAR-) over (= reversal indicator) column (-ROW-) and took particular attention to (-MINDED) (NARROW-MINDED) the type that’s ultra-conservative (NARROW-MINDED),
6 Dump nosey (dump nosey = PSEUDONYM) model (= anagram indicator) making a new name for herself (PSEUDONYM),
7 State (= homophone indicator) did a job for the bakers (“kneaded” = “NEEDED”) when it was required (NEEDED),
8 Hang out loosely (DANGLE) with Glenda (Glenda = DANGLE) Duff (= anagram indicator),
11 Nicer type (KINDER-) cooks (= anagram indicator) great (great = GARTE-) nibbles primarily (‘nibbles’ primarily” = -N) (KINDERGARTEN) for the little ones before school (KINDERGARTEN),
15 Gave grounds for (JUSTIFIED) gravy (JUS-) good enough (fit = -TIF-) to serve up (= reversal indicator) with starters in edible dishes (starters “in edible dishes” = -IED) (JUSTIFIED),
16 Type of bottle (SCREW TOP) that’s not made in Cork (SCREW TOP not made in cork),
17 Likes to share (FLATMATE) with novice (learner = -L-) consumed by stout (F-AT-) and Bud? (-MATE) (FLATMATE),
19 Sheepish type (LAMB-) in the family (-DA) (LAMBDA) gets letter from Greece (11th Greek letter = LAMBDA),
20 Is (Is = SI-) setting up (= reversal indicator) train (= anagram indicator) set (set = -EST-) from midday (mid ‘day’ = -A) (SIESTA) for the rest of the afternoon in Madrid (SIESTA),
23 Skeleton (DRAFT) key (music key D-) found on river transport (-RAFT) (DRAFT).