Ok, so its not an iron and polishing board is a more appropriate term than ironing board, but they are for smoothing, pleating and polishing linen.
These form part of the grave goods from a Viking era burial of a wealthy woman at Birka, in Sweden. The board is carved whalebone and the smoother is made from glass. They can be seen, along with other exquisite items from her grave, on display at the Museum of National Antiquities in Stockholm (Historiska Museet), Sweden.
The Romans used smooth lumps of glass for pressing linen too.