A day or two ago while surfing the Internet, I came across the following article from the Proceedings of the Society of Antiquaries of Scotland: Paterson, Caroline. "The Viking Age trefoil mount from Jarlshof: a reappraisal in the light of two new discoveries." Proc. Soc. Antiq. Scot. 127 (1997), pp. 649-657. The link here goes to a page where one can download the PDF.
The gist of the article is that the Jarlshof trefoil, which is broken and only about two-thirds complete, is very similar to two complete trefoils found in the British Isles. The British specimens, unlike typical trefoil brooches, have three sets of brackets on the back (one on each point of the trefoil), instead of one for the hinge of a pin and one for the point. Paterson suggests that the Jarlshof mount was made in Great Britain, and that it may not have been used as a brooch but may have been (as the early items that inspired the design in the first place were) a strap distributor.
It seems to me that these trefoils suggest some interesting things about contacts between Scandinavia and Great Britain that historic costumers should bear in mind.