Across:
1 Made one’s mark (SIGNED) with dress (= anagram indicator) design (design = SIGNED),
4 Most of (e) the dish (‘plate’ without ‘e’ = PLAT-) on paper (-FORM) (PLATFORM) is a level above the rest (PLATFORM),
9 Meeting leaders perhaps (CHAIRS lead meetings) misrepresented (= anagram indicator) as rich (as rich = CHAIRS),
10 Picked up (OBTAINED) first of the onions (first of the “onions” = O-) that ain’t (aint = -TAIN-) fresh (= anagram indicator) coming out of plot (-B-ED) (OBTAINED),
12 Nothing (NAUGHT-) to do with Irish (-I-) and English (-E-) writing (from the 3Rs = -R) (NAUGHTIER) is more mischievous (NAUGHTIER),
13 Types (SORTS) of forecasts that lose face (‘forecasts’ that lose ‘face’ = orsts = SORTS),
14 Took a seat (SAT-) from one (-I-) small (-S-) pressure group (-FACTION) (SATISFACTION) with sense of achievement (SATISFACTION),
18 Pear tree must (pear tree must = TEMPERATURES) develop (= anagram indicator) in various degrees of heat (TEMPERATURES),
21 Gave a lecture (SPOKE) on the bike (SPOKE),
22 Citadel in (citadel in = IDENTICAL) ruin (= anagram indicator) is the very same (IDENTICAL),
24 Dry (AIR-) fortified wines (-PORTS) (AIRPORTS) for those in lounges preparing to take flight (AIRPORTS),
25 Sweetheart (lover = REVOL-) reverses (= reversal indicator) car (model T = -T) (REVOLT) in disgust (REVOLT),
26 Negative aspect (DOWNSIDE) to team in Ulster (DOWN-) and Leinster, for example (-SIDE) (DOWNSIDE),
27 Made a play for (CHASED) amateur (-A-) society (-S-) in revolutionary (CH-E-) first drama (first ‘drama’ = -D) (CHASED).
Down:
1 An issue for the HSE? (SICKNESS) One is better off without it! (SICKNESS),
2 From an educational prospective, master (GRADUATE) butcher (= anagram indicator) argued (argued = GRADU-E) over a (-A-) bone (-T- bone) (GRADUATE),
3 Source in The Yard (EARTH) keeps distance from The Sun? (EARTH).
5 They have a reputation for employing rats (LABORATORIES) in the French (the = LA-) Bora-Bora? A fair amount (half of) (half of ‘Bora-Bora’ = -BORA-) of politicians! (-TORIES) (LABORATORIES),
6 Vehicle (TRANSPORT) for first team (first ‘team’ = T-) managed (-RAN-) by school head (‘school’ head = -S-) from coastal city (-PORT) (TRANSPORT),
7 Lift lid off (d) casual drugs (‘downers’ without ‘d’ = OWNERS) for those legally in possession (OWNERS),
8 Humble (MODEST) Wordsworth works (-ODES-) in Montana (M-T) (MODEST),
11 Upset (DISAPPOINTED) German (D-) is (-IS-) given the contract (-APPOINTED) (DISAPPOINTED),
15 Criminal (= anagram indicator) stole Ken’s (stole Ken’s = SKELETONS) remains (SKELETONS),
16 Players (-ACTORS) almost (y) score (‘try’ without ‘y’ = TR-) from the start (= positional indicator) (TRACTORS), breaking new ground across the turf (TRACTORS),
17 Does tail (does tail = ISOLATED) wag (= anagram indicator) in quarantine? (ISOLATED),
19 One (I-) school (-S-) near Limerick (-L-) and (-AND) (ISLAND) Clare, for example? (Clare ISLAND),
20 Plagiarise (rob = BOR-) review (= reversal indicator) and column (-ROW) (BORROW) and take credit? (BORROW),
23 Central to committee thinking (central to ‘committee thinking’ = tee th = TEETH) these are important to the consumers (TEETH).