Across:
1 Opine (opine = PI-EON) all over the place (= anagram indicator) about Government (-G-) (PIGEON) in The Messenger (PIGEON),
4 Clip of (m) confidence trick (‘scam’ without ‘m’ = SCA-) by political party (Fianna Fáil = -FF-) is not new (-OLD) (SCAFFOLD) – it’s the one where the wrongdoers get suspended (SCAFFOLD),
9 Without a leader (m), mob (‘mob’ without ‘b’ = OB-) watched (-EYED) (OBEYED) and followed (OBEYED),
10 Rough (= anagram indicator) partisan (partisan = ASPIRANT) has ambitions (ASPIRANT),
12 Yellow-green shade (PISTACHIO) of Italian river (River Po = P-O) going around landscape (vista) without heading (v) (‘vista’ without ‘v’ -ISTA-) for Switzerland (-CH-), for one (-I-) (PISTACHIO),
13 Workout (work out = SOVLE) with heartless (l) Green type (‘olive’ without ‘l’ = -OLVE) after (= position indicator) school (S-) (SOLVE),
14 Seek office (CONTEST A SEAT) for campaign (CONTEST A SEAT),
18 Taking the car (DRIVING) to the mountains (RANGE) (DRIVING RANGE) popular with the swingers (DRIVING RANGE),
21 Frenchman (Monsieur = -M-) in part (part = TRA-P) upset (= reversal indicator) (TRAMP) by hike (TRAMP),
22 Spud’s (spuds = ROOT CROPS) motherland (roots = ROOT -S) covers most of 23 down (most of ‘crops’ = CROP-) (ROOT CROPS),
24 Writing (from the 3Rs = R-) with journalist (-E-D) about charter (-LEASE-) (RELEASED) in what turned out to be Deliverance (RELEASED),
25 Reserve (BOOK-) that is (i.e. = -IE) (BOOKIE) doing all his work on the pitch, of course? (BOOKIE doing all his work on the pitch at the racecourse),
26 Part payments (DEPOSITS) for butcher’s (= anagram indicator) topsides (topsides = DEPOSITS),
27 Liveliness (ESPRIT) in the ultimate Champagne (the ultimate ‘champagnE’ = E-) fizz (hiss/spit = SP-IT) found around river (-R-) (ESPRIT).
Down:
1 Pops cert (pops cert = PROSPECT) out (= anagram indicator) in anticipation (PROSPECT),
2 Trying to answer the riddle (GUESSING) of scrambled (= anagram indicator) eggs in US (eggs in US = GUESSING),
3 Upsets (= reversal indicator) American (-A) agent (rep = -PER-) linked to fair amount of (half of) (half of ‘Of’ = O-) (OPERA) musical drama (OPERA),
5 Swear local (‘local’ cuss = CUS-S) rings chap (-TOM-) from Labour’s working group (UNION) (CUSTOMS UNION) agreeing similar duties (taxes) for the members (CUSTOMS UNION),
6 Be reasonable (FAIR’S FAIR) with ticket prices (fares) on the radio (= homophone indicator) (‘fares’ = ‘fairs’ = FAIR’S) for festival (FAIR) (FAIR’S FAIR),
7 Know all (know-all = ORACLE) the old (O-) Irish, for instance (race = -RAC-E), around Limerick (-L-) (ORACLE),
8 Sit on the fence (DITHER) with woman (-HER) after (= positional indicator) college (Dublin Institute of Technology = DIT-) (DITHER),
11 Who cares (WHATEVER) what the cashier says (NEXT) (WHATEVER NEXT), as it comes as a surprise (WHATEVER NEXT),
15 A race (a race = EUROPEANS) for the locals you’d associate with Dubliners, for instance (EUROPEANS),
16 Individual (ON-E-) covering fashion (fashion = -LOOK-) from centre of Paris (from centre of ‘paRis’ = -R) (ONLOOKER) for The Observer (ONLOOKER),
17 Arrests (arrests = REASS-RT) criminal (= anagram indicator) outside Spain (-E-) (REASSERT) for State again (state again = REASSERT),
19 Rudely studied (STARED) celebrity (STAR-) writer (-ED) (STARED),
20 What the Arabian might do (Arabian horse might GALLOP) left (-L-) in foreign (= anagram indicator) gaol (gaol = GA-LO-) by political leader? (‘Political’ leader = -P) (GALLOP),
23 Edits the picture (CROPS) of star getting out of sports car (‘star’ getting out of ‘sports car’ = porsc = CROPS).