Rob Penney made 14 changes to the side that had beaten Scarlets the previous Saturday, with Hanrahan the only player to make the cut.
And the fly-half did not disappoint, sneaking in for a first-half try and navigating the conditions capably to kick 17 points from the tee.
Ulster slipped from second to fifth with defeat at reigning RaboDirect PRO12 champions Leinster, as Jordi Murhpy ran in the game's only try.
And Jimmy Gopperth notched 14 points with the boot as Leinster leapt to second in the table.
Glasgow Warriors came out on top in the first of the back-to-back 1872 Cup clashes, as Stuart Hogg touched down to break Edinburgh hearts.
Dougie Fife had scored for the hosts but five Duncan Weir penalties brought Glasgow back into the contest, before they struck the decisive blow.
Ospreys and Newport Gwent Dragons triumphed in a Boxing Day double of Welsh derbies, beating Scarlets and Cardiff Blues respectively.
Ospreys won a low-scoring clash at Pary y Scarlets, while Toby Faletau shone as the Dragons ended a run of poor form.
And Benetton Treviso came out on top in Saturday's all-Italian contest, as Cornelius van Zyl and Manoa Vosawai scored to hand Marius Goosen his first win since taking charge.
TRY OF THE WEEKEND
Glasgow Warriors extended their hold over rivals Edinburgh in the 1872 Cup and it was a late moment of magic that sealed the win on Boxing Day.
Trailing by one point, the Warriors spread the ball wide inside their own half until it found inside centre Alex Dunbar.
Dunbar, who had signed a new contract at Scotstoun earlier that week, broke two tackles before offloading to DTH van der Merwe.
Hugging the left touchline, the winger kicked deep into the Edinburgh 22 and the onrushing Stuart Hogg collected the perfectly-weighted chip to touch down the winning try.
BREAK OF THE WEEKEND
Though they did go on to clinch the Welsh derby on Boxing Day, Ospreys were denied a vital second-half try after an impossibly tight TMO decision.
The move began with Alun Wyn Jones collecting a lineout on the Scarlets' ten-metre line and handing the ball off to scrum-half Rhys Webb.
Dan Biggar then orchestrated a clever overlap before finding Sam Davies and the full-back sent Hanno Dirksen streaking clear down the right.
Dirksen ran 40 yards and dived for the line but three Scarlets defenders were adjudged to have just done enough to prevent the opening try.
PLAY OF THE WEEKEND
Leinster ground out a result against fellow Irish outfit Ulster on Saturday with a determined display, but the game's only try was a spark of genius.
The hosts were trailing 6-3 shortly before the break when a sloppy lineout fell to Luke Fitzgerald 30 yards from the Ulster line.
The winger ran straight into the visiting pack, breaking a tackle and offloading in the contact to No.8 Jordi Murphy.
And the man of the match did the rest, powering home in the corner despite the close attention of two Ulster defenders.
PLAYER OF THE WEEKEND
In the week's second Welsh derby Newport Gwent Dragons piled the misery on Cardiff Blues and, more often than not, it was Toby Faletau leading the charge.
The British & Irish Lions No.8 touched down in the first-half for the hosts' only try, sneaking around the front of a Dragons lineout to canter home unchallenged.
And from then on he was a constant thorn in the Blues' side, forcing infringements at the breakdown and leading the resilient defence at the death.
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