Across:
8 Is rare (is rare = SIERRA) cuckoo (= anagram indicator) from a range of mountains? (SIERRA),
9 Opening 15 across (opening ‘devil-worshipper’ = DEVIL-) is (-IS-) initially hardly (initially ‘Hardly’ = -H) (DEVILISH) diabolical (DEVILISH),
10 Call (call/bleep = PAGE) one being studied in 12 across possibly (PAGE being studied in ‘gospel’),
11 This ironic (this ironic) adaptation (= anagram indicator) (this ironic = HISTRIONIC) is melodramatic (HISTRIONIC),
12 Good book (GOSPEL) but not one for 15 across (GOSPEL not for ‘devil worshipper’),
14 European (CROATIAN) actor in a (actor in a) play (= anagram indicator) (actor in a = CROATIAN),
15 Sings the praises of the man downstairs (DEVIL WORSHIPPER) for opening 22 across (opening ‘devil to pay’ = DEVIL-) at mass meeting (WORSHIPPER) (DEVIL WORSHIPPER),
18 Unworried (CAREFREE) by 24 down (‘care’ = CARE-) and 2 down (‘free’ = -FREE) (CAREFREE),
20 Sounds like (= homophone indicator) something you’d listen to on a record (‘B side’ = ‘BESIDE’) is close to Elbow? (at someone’s elbow = BESIDE),
22 Severe repercussions (DEVIL TO PAY) for most of (s) 21 down (most of ‘devils’ = DEVIL) approaching Cork (top = TO P-) and primarily (‘And’ primarily = -A-) half of Offaly (half of ‘OY’ = -Y) (DEVIL TO PAY),
24 Some eccentric – a vegetarian (some ‘eccentriC A VEgetarian’ = CAVE) left holding the wine (CAVE),
25 Pitiful (PATHETIC) Irishman (PAT-) with work (= anagram indicator) ethic (ethic = -HETIC) (PATHETIC),
26 Sharp (SHRILL) slope initially (‘Slope’ initially = S-) for the Dubs (the Hill = -H- ILL) overlooking river (-R-) (SHRILL).
Down:
1 Five (5 = V-) paramilitaries (-IRA-) leave (-GO) (VIRAGO) in Spitfire (spitfire/bad-tempered = VIRAGO),
2 Life peer loses pile (‘life peer’ loses ‘pile’ = feer = FREE) that’s unoccupied (FREE),
3 Milk producer (cow = C-OW) takes in part of the forest (-ASH) near Florida (FL-) (CASH FLOW) with money that’s readily available (CASH FLOW),
4 Leaderless (s) factions (‘sides’ without ‘s’ = IDES) in Rome warned about them (IDES of March warning to Caesar),
5 Amplify the fact (OVERDO) old (O-) composer (Verdi) isn’t finished (i) (‘Verdi’ without ‘i’ = -VERD-) with orchestral leader (‘Orchestral’ leader = -O) (OVERDO),
6 Presumably, you’d expect The Count to get his teeth into these (Count Dracula get teeth into BLOOD TYPES) for BA? (B+A = BLOOD TYPES),
7 Set (set) play (= anagram indicator) (set = EST-) from one (-I-) friend (-MATE) (ESTIMATE) – a ballpark figure (ESTIMATE),
13 The sort of thing that happens in Ring (PRIZEFIGHT) with Jimmy (jimmy/force = prise) on the radio (= homophone indicator) (‘prise’ = ‘PRIZE’) and box (FIGHT) (PRIZEFIGHT),
14 Doctor (= anagram indicator) cures (cures = CURSE) with bitter pill (CURSE),
16 Dave (Dave) retires (= reversal indicator) (Dave = EVAD-) at end of surveillance (end of ‘surveillancE’ = -E-) duty (TAX) (EVADE TAX) to work in the black economy (EVADE TAX),
17 The Anchor (host = HO-ST) over hostelry (B&B = -BB-) closes for cinematography alumni (closes for ‘cinematographY alumnI’ = -YI-) (HOBBYIST) – the one interested in Trainspotting, for instance (HOBBYIST),
19 Take turns (ROTATE) to judge (R-ATE) good books (Old Testament = -OT-) inside (ROTATE),
21 Director (D-) from Elvis (Elvis), Rocky (= anagram indicator) (Elvis = -EVILS) (DEVILS) and Spooks? (DEVILS),
23 Ends war (PACT) amidst group activism (amidst ‘grouP ACTivism’ = PACT),
24 Get it from cigarette (‘get it’ from ‘cigarette’ = care = CARE) and that’s not neglect (CARE).