The IRA have had a month to dig in at Athlone, even if they are only irregulars with advisors the UVF isn't much better. Assaulting such positions without artillery is not a good idea, even if they are victorious there will not be much UVF remaining afterwards. (Not a regrettable outcome from a political perspective when you think about it.) If the German regulars reach Athlone before the UVF ... well.