Another simple tip: ask the police!
Here's one for you: did you know that if you're stuck outside Lansdowne Road on matchday without a ticket, there's a very good chance of you getting a ticket from a Garda (Irish policeman)?
Yes, seriously. Croke Park too for that matter, but that's for another day.
Back in 2002, from a Garda outside, I got a ticket for a really big sold out game - I think it was France - just after kick-off. Yes, I said "got" - he gave it to me for free!
Perseverence pays off!