Angus Martyn, Freddie Lait and Graham McCallum went over for Muir - the first two from breakaways.
Ross Cook added the rest of the points with the boot.
Lock Damien Kelly snapped up Ayr's opening score, the other being a penalty try, with Nick Lavelle slotting two conversions and two penalties.
Glasgow Hawks outpowered Dundee HSFP 19-11 in a dour and jittery tussle at stormy Mayfield.
Neither team looked capable of stringing together sequences of flowing rugby, but toppled champions Hawks just deserved their 19-11 win.
Greg Francis was the men who mattered most, claiming two tries, the others being scored by Stevie Gordon and Murray Strang.
Neil McComb gathered the Taysiders' only touchdown, while Barry Jones kicked a conversion and penalty.
The match which also featured the first joint appearance of former Glasgow star Rory Kerr and brother Jamie.
Aberdeen GSFP look destined to drop out of the top division in the wake of their tense 24-21 defeat by Watsonians at Rubislaw - coupled with Hawick's triumph over Heriot's.
But it was a brave display by the Dons, who will play a lot worse and win games.
They were in with a great chance of grabbing a lifeline when they led by two points going into the final stages, only for left wing Ally Rowe to clinch the glory for Sonians, keeping their slim title dream alive.
The other Watsons counters came from player-coach Cammy Mather, Mike Ker and James Easton, deep in first-half stoppage time. Ker kicked the remainder.
A burst of touchdowns from Sean Crombie and Grant Strang gave Aberdeen fresh momentum, with Ben Spearitt landing a penalty hat-trick.
But Rowe had the last word to leave them fighting for their top-flight existence.
Currie took another stride towards their first Premiership crown - thanks mainly to the stamina of their pack.
Currie coach Ally Donaldson had branded the trip to Melrose as their highest hurdle on the home straight of the campaign.
And his side were forced to battle back from an interval deficit to keep up their championship surge with a 15-12 win.
The crucial phases of play spanned the break when Melrose were down to 13 men thanks to the sin-binnings of back-row pair Graham Dodds and Andy Thomson for killing the ball.
Although the Borderers maintained their 12-3 advantage at half-time, they leaked a try immediately after the interval which handed Currie the momentum at a crucial stage.
Tries by wingers Alex Jessop and Calum Anderson, along with a Craig Edwards conversion, gave Melrose the early initiative.
But Currie's backlash featured touchdowns from Chris Brown and Mark Cairns, while Ally Warnock added a conversion to his first-half penalty.
Hawick dominated their mud-strewn encounter with Heriot's but had to cling on for an 11-7 success at Mansfield Park.
Dan Landels pounced for the only Greens' try, while Rory Hutton banged over the two penalties which proved vital.
The Heriot's touchdown was bagged by Graham Scott, goaled by Graham Wilson.
But, on the balance of play, Heriot's were fortunate to leave with a consolation bonus point.
Premiership One
Aberdeen GSFP 21 - Watsonians 24
Boroughmuir 20 - Ayr 20
Dundee High FP 10 - Glasgow Hawks 19
Hawick 11 - Heriot's 7
Melrose 12 - Currie 15
Premiership Two
Stirling won the top-of-the-table clash in Premier Two, defeating GHA 13-9 thanks to scores from Bruce McFarlane and Ross Aitken with Brian Archibald landing a penalty. Stirling are now nine points ahead of Edinburgh Accies, who move above GHA after a 22-17 win over Selkirk.
Hamilton leapfrog Stewart's-Melville after their battling 13-12 win at Laigh Bent, despite trailing 0-12 at half-time to tries from Alex Cox and Sean McMurtry, and a Richard Borthwick conversion. The Hamilton fightback started with a Kenny Diffenthal penalty. Craig Sangster followed up with a try and Diffenthal's try was the clincher for the home side. Meanwhile, at Hartreemill, Biggar and Cartha QP shared the points in a 14-14 draw.
Biggar 14 - Cartha QP 14
Edinburgh Accies 22 - Selkirk 17
GHA 9 - Stirling County 13
Gala 10 - Jed-Forest 8
Hamilton 13 - Stewart's Melville FP 12
Kelso 8 - Hillhead-Jordanhill 12
Premiership Three
Former Glasgow and Scotland captain Gordon Bulloch scored three as West Of Scotland beat Ellon 24-13 to cut Musselburgh's lead at the top of Premier Three to a single point. Johnny Sloan scored West's other try.
Musselburgh could only muster a draw against third-placed Dunfermline despite having a 6-3 half-time lead courtesy of two Angus Stewart penalties. However, two Ally O'Connor penalties and a Chris Richards drop-goal to a third Stewart penalty ensured the spoils were shared.
Berwick 12 - Haddington 18
Cambuslang 14 - Peebles 24
Dunfermiline 9 - Musselburgh 9
Ellon 13 - West Of Scotland 24
Murrayfield Wanderers 15 - Livingston 23
Perthshire 25 - Kirkcaldy 42