The 109 is a speedy little scooter and may very well have come up on the Tower freq *after* the Lear had been cleared for takeoff (that is, if the Irish are in the habit of clearing you for takeoff before you actually get into "position"- as happens frequently here in the U.S.). So there might be a time lag there.
Nevertheless, if the 109 pilot assumed that Tower meant the 737 taxiing out, then he certainly violated that portion of his clearance by proceding across the runway in front of said traffic. So they evidently weren't paying attention.
TCAS might *not* have helped here because the Lear pilots may have waited until they were actually on the runway before switching their transponder on.
But let's not forget about the Lear! Do they not have a responsibility to *look* and make sure their departure path is clear? Did neither of them look out the windscreen and happen to see a sparkly thingee approaching the runway from off-side? And...even if they'd already been cleared for takeoff, didn't *they* hear the Tower clear the 109 to cross behind them?
Just how busy *is* Dublin Tower, anyway? Curious, I punched up liveatc.net and listened in today at around 1900 local Dublin time. Eh- wasn't all that busy on a clear Friday night.
These things happen - they shouldn't but they do. They're embarassing. And they're the reason we all must be vigilant 110% of the time, especially when operating around airports.
Parenthetically, in New York the controllers always used took us over the approach end of the runways we were crossing. Seemed like a practical idea. I've been surprised in other cities when Tower has cleared me to cross an active runway "wherever." When I've suggested whether they wanted me over the numbers or not, they didn't seem to care at all. Like New Orleans, for instance. Very laid-back there.