CO don't publish any fares DUB-TYO, I would guess because they don't see a market for them.

The S fares (DUB-HOU-DUB & HOU-TYO-HOU) are not combinable on the same ticket, but there's nothing to stop you buying two separate tickets. Only two caveats:

- you may not be able to check bags all the way through (e.g. TYO-DUB via HOU and EWR) if using separate tickets
- you may not be protected if, say, you were connecting at HOU and the inbound flight were late, causing you to misconnect.

So, build in a safety buffer if you're planning to connect directly through HOU without a stopover.

hth