Do people seriously think he dopes? I won't rule it out, but it seems highly unlikely. What would be the incentive? It's not like a 60 year-old masters runner from Ireland is going to be a sponsorship magnet or that there's any sort of decent prize money so there's no financial gain. There's no real fame either, beyond a few notices in the running community.
To me, he's just a very good athlete who, for some reason, has stayed demotivated into later life in a way that other Olympic athletes have not. I'm almost certain someone like Steve Jones could have run quite a lot faster than 2:30 in his 60s if he had the motivation. Tommy is just one of the very few Olympic level athletes that wanted to keep running hard into their pensionable years