With reservations and no guarantees given I manged to find one possible maker punching like this (IR). But before I tell you the name, I'd like to draw your attention to why I believe it is this maker. The twisted/spiral stem and the style, no to talk about the fineness of 13¼ lot indicate Norway around 1850. In Christiania (Oslo) I found Johan Martin Heinrich Rohlff, born 2.8.1850 in Copenhagen/DK. He moved to Christiania/NO 1. April 1844 and worked there as apprentice with F. J. Brick and journeyman as from 23.4.1850. He became master 19.10.1856 and was active to 1900. He dies 1903 or 1904. He made spoons among other things. By the way, he also punched I.ROHLFF.
Is it him or not is up to you to decide ;-)
PS. There are 2 IRs in Denmark, but in my opinion both are too early working in the 18th century.