No Irish soldier can join another army.. No. Basically the members of the Irish Defence Forces, can and did in the past transfer to the Gardai Siochana (Irish Police) and the Prison service. But...
You need to be a Canadian citizen to join the Canadian Forces.
No, Ireland and Britain are different countries, you would have to leave the Irish Army, then try to get permission to join the British army
8,500 plus around 11,000 reservists.
Quite impressively during WW2 Australia with just a population of 7 million people raised an army of 1.2 million soldiers.