Across:
1 In retrospect (= reversal indicator), research site (lab = -BAL-) divides Frenchmen (MM = -M-M) initially eager (initially ‘Eager’ = E-) (EMBALM) to use preservatives (EMBALM)
4 Veteran (veteran = REVENA-T) travelled (= anagram indicator) around north (North = -N-) (REVENANT), like one returning from the dead (REVENANT),
9 One expected to turn back the tide (CANUTE) by serving up (= reverse indicator) fish (tuna = -ANUT-) caught in the community (European Community = EC = C-E) (CANUTE),
10 Rival (OPPONENT) of old (O-) father (parish priest = -PP-) figure (-ONE)-) from good books (New Testament = -NT) (OPPONENT),
12 Son of a promoter opens (‘Son Of A Promoter’ opens = SOAP) musical with diva (OPERA) (SOAP OPERA) from never-ending story of real-life events (SOAP OPERA),
13 Praise (EXTOL) former (EX-) leads on The Onedin Line (leads on ‘The Onedin Line’ = -TOL) (EXTOL),
14 Perhaps still (not fizzy), it’s a healthy drink (MINERAL WATER) found in spring (MINERAL WATER),
18 Runs the school (HEADMISTRESS) in Schull (Schull) it’s said (= homophone indicator) (‘schull’ = ‘skull’ = HEAD-) for the other woman (-MISTRESS) (HEADMISTRESS),
21 It’s heart-warming without harming (‘heart-warming’ without ‘harming’ = etwar = WATER) what divides the Irish and the English (WATER),
22 Lifts the lid on what’s preserved (CAN OPENER) in vehicle (CA-R) containing number (-N O-) for writer (-PEN-) from Spain (-E-) (CAN OPENER),
24 Exaggerates (OVERACTS) what the player does (-ACTS) after (= position indicator) Romeo (lover) loses the head (l) (‘lover’ without ‘l’ = OVER-) (OVERACTS)
25 Mystery (ENGIMA) of wild (= anagram indicator) game (game = E-GMA) found around Ulster (-NI-) (ENIGMA),
26 Prop (SET-) from America (-A-) joins Munster (-SIDE) (SET-ASIDE) as reserve (SET-ASIDE),
27 Backs (= reversal indicator) driving aid (tee = -EET-) coming from port (port/left = -L-) in Belgium (BE = B-E) (BEETLE) by car (BEETLE).
Down:
1 Let off (EXCUSE) for first murder? (first ‘Murder’ = M-) It’s an eye-opener (‘Eye’ opener = -E) (EXCUSE ME) begging your pardon! (EXCUSE ME!),
2 Outlaw (BAN-) and top dog (top ‘Dog’ = -D-) takes head off (c) big showy flower (canna) (‘canna’ without ‘c’ = -ANNA) (BANDANNA) – the one that’s used to cover the head (BANDANNA),
3 Dismiss (LET GO) those starting (‘Those’ starting = -T) in the building game (lego = LE- GO) (LET GO),
5 Clarifying statements (EXPLANATIONS) by old lover (EX-) and pal (pal) travelling (= anagram indicator) (pal = -PLA-) in Europe, for instance (-NATIONS) (EXPLANATIONS),
6 It appeals to a couple of beloved runners (ELOPEMENT) getting away for the match (ELOPEMENT),
7 Evangelist gets rid of evil (‘evangelist’ gets rid of ‘evil’ = angest = AGENTS) business managers (AGENTS),
8 Count’s (counts = TOTALS) last to (last to) explode (= anagram indicator) (last to = TOTALS),
11 Has a lot of knowledge (WELL-EDUCATED) of where you’d expect to find 21 across (expect to find ‘water’ in WELL-) on train (-EDUCATE-) going to Dublin (-D) (WELL-EDUCATED),
15 Right (R-), of course (yes = -YE -S), about head of bioterrorism (head of ‘Bioterrorism’ = B-) getting to study (-READ-) (RYE BREADS) what’s produced by those with specific dough (RYE BREADS),
16 Time to retire (NIGHT) after (= position indicator) foul (foul) broadcast (= homophone indicator) (‘foul’ = ‘fowl’ = HEN) (HEN NIGHT) of social that appeals to one joining a two-party system? (HEN NIGHT),
17 One taking drugs (USER-) from celebrity (-NAME) (USERNAME) with online identity (USERNAME),
19 Is overcome with adoration (SWOONS) for small (S-) court (-WOO-) in front of (= position indicator) school (National School = -NS) (SWOONS),
20 Confirm (ATTEST) strange (= anagram indicator) taste (taste = ATTES-) is close to repugnant (close to ‘repugnanT’ = -T) (ATTEST),
23 Writer (pen = PE-N-) consumes the last of the Parmesan (last of ‘parmesaN’ = -N-) on top of egg (top of ‘Egg’ = -E) (PENNE) pasta (PENNE).