http://www.joyofbaking.com/WelshCakes.html says ground cinnamon, mace, currants and candied citrus peel

If you mean the small round teacakes with currants, the traditional spice to add is mace, but nutmeg isn't a bad substitute.

I like a combination of allspice and cardamom to accent the currants. Nutmeg is also good, especially if you serve them with blueberry jam.