Is there a set for the Williamite Wars in Ireland?

There is no Polemos set for this, although you could retrofit the WSS set.   The GNW set is also a possibily as this takes pikes into account, however, it may require a little more modification as it has a very Northern and Eastern European take on the period.

Have a look at the 'Twilight of the Sun King' rules. The basing and unit sizes used in these rules are very suitable for 6mm - I'll be using their sister-set, 'Twilight of the Divine Right' for my Baccus Thirty Years Wars armies (when I've finished painting my 1914 Germans & Russians!!!!)

I bought my copies through the Pike and Shot Society. I think they still sell them, along with the scenario books which do contain at least one for the Williamite Wars.