The British in Egypt wore round hats because of the bright desert sun, a fairly rare phenomenon in Ireland. For the '98 rebellion troops were suffering from Dundas' fascination with all things Frederickan. They should be wearing full-tailed coats, double breasted but closed to the waist, with stand-up collars. They should be in true bicornes (as opposed to the cocked hats of the 1793-94 campaign). They wore white breeches and high black gaiters. The closest thing available would be the old ESCI Prussians. If they are unavailable then the HaT 1806 Prussians would do in a pinch, despite the trousers.
The French troops sent to aid the rebels wore a chapeau allemagne, i.e., a broad-brimed, round-crowned hat turned up on the left side. The Accurate/Revel/IMEX American Militia for AWI would do for the rebels. Their dress is rather old-fashioned but then rural folks tend to be behind the fashion curve. They could be fleshed out by the IMEX Alamo defenders, especially those in top hats and frock coats.
The rebels were short of firearms and resorted to "half-pikes" which were simply thrusting spears. Any figure charging or firing from the waist (as can be found in the Alamo Defenders) could have their firearm replaced by a suitable polearm.