The hooker’s second half brace saw Ireland pull away after an evenly-matched first period ended 13-7 in their favour, Sophie Barrett crossing early and Kate Flannery’s boot adding to the lead before Lucia Scott’s try put Scotland back in it heading into the break.
There was no stopping Buttimer after the interval and she crossed for two close-range tries either side of scores from Eadaoin Murtagh and Hannah Wilson.
Ireland finish the competition with a victory and two defeats from their three games, while Scotland return from Parma winless.
Ireland started on top and were ahead inside six minutes when prop Sophie Barrett powered over from close range for a score converted by Flannery.
Scotland responded well and enjoyed a couple of visits to the Irish 22 without success, Eilidh Fleming benefiting from an Ireland overthrow to make good ground before Ceitidh Ainsworth shimmied her way through only for her side to be penalised at the breakdown.
Ireland thought they had a second try when Clara Barrett collected Flannery’s long, looping pass and finished in the left corner but the pass was ruled to be forward. With a penalty advantage in their pocket, Ireland went for a tap and go but their pack were held up over the line by dogged Scottish defence.
Two Flannery penalties extended Ireland’s advantage to 13-0 either side of superb defence from captain Jane Neill, who got a hand underneath the ball to keep Scotland at bay when Gemma Bell powered over the line.
Both sides lost players to injuries in an elongated first half, Ainsworth departing for Scotland and Kelly Burke for Ireland, and both sides were held up over the line again before the whistle was blown.
Scotland were not to be denied in the final action of the half, however. A quick tap from a penalty saw the pack unable to find a way through but when the ball went through the hands, Lucia Scott was on hand to apply the finish following a neat step inside.
The second half began with Ireland on the front foot and Scotland needed to defend for their lives to keep them out initially, holding their opponents out under the posts in the 44th minute.
But Claire Cruikshank’s side missed an opportunity to clear their lines and gain field position when a lineout was knocked on inside their own 22 and Ireland made them pay when Murtagh dotted down following a catch and drive.
Buttimer stretched the lead shortly afterwards as Ireland’s pack turned the screw and she was at it again to complete a quickfire double, finding a gap in a mass of bodies to make the game safe.
Scotland were reduced to 14 players in the closing stages following a yellow card and Ireland made their numerical advantage count when Wilson spun and touched down to the right of posts with seven minutes to play.
Ellen O’Sullivan-Sexton added the extras to put the finishing touches to an impressive Irish display and ensure they ended their time in Italy on a high.