The next obvious thought was the cavalry, as there are several "Guards" cavalry regiments, Life Guards, Royal Horse Guards, Dragoon Guards etc
Only 2 Irwins in the cavalry though: 2nd Lieut Thomas Irwin, 1st Dragoons & Pte W.Irwin, 3rd Dragoon Guards. On the whole not promising for you.
The Imperial Yeomanry look a bit more promising.
31970 Pte J.A.Irwin
33251 Pte Robert Irwin
31975 Pte William Irwin
all 61st (2nd Dublin) Coy, 17th Btn I.Y
IY service records would hopefully confirm if they were brothers (or not).
3 Irwins and not 4, but the best I have.
