A 30 minute window is the norm in Dublin anyway, though I have seen some that run up to an hour. The listings in MyHome or Daft will state start/finish times if it's an open viewing. Particularly on Saturday mornings, agents are usually rushing off to the next one, so they don't hang around.
If you are interested in a property to the point of bidding, the agent will usually accommodate a further unadvertised viewing where you or a surveyor/ engineer you appoint (at your expense) can spend a little more time assessing the work to be done.