heathrow airport (terminal 1 to terminal 3...要多少時間?急!!!

Well... I suppose you mean the transit time at London Heathrow... If you have check-in all the way to Hong Kong from Scotland... The transit time would be 1hours 30minutes (and that include arriving at your boarding gate at Terminal 3 20minutes before departure...)... But in my experience, the process is usually around 1hour, but when you include the possibility of flight delay, the standard 1hour 3ominutes could be a little tight...
With the enhanced security measure at Heathrow... You will require a similar security check while transiting at Heathrow... And this process could be rather timely if there is a huge amount of transit pax... So I would suggest you to leave at least 2hours for transit between flight... And if you happen to be lucky enough to experience smooth process at Heathrow, by the time you arrive at Terminal 3, you should have the gate information for your flight to Hong Kong already... Just that it may not be boarding time yet...
So according to British Airport Authorty, the standard minimum transit time that they suggest is 1hour 30minutes... But I would suggest you to allow a little bit of extra time because of the extra security measure to leave around 2hours at least for transit...

2007-01-28 05:30:30 補充：
Sorry about that... Yes it should be Ireland... But the process is the same... And the transit time is still the same no matter where you are connecting from... And no matter which terminal you are in... The standard transit time is at least 1hour 30minutes...

"Terminals 1, 2 & 3 are all located in the centre of the airport and are within walking distance of each other via underground walkways.
Terminal 4 is located on the south side of the airport, and can be reached from Terminals 1, 2 & 3 via the free inter-terminal transfer service on the Heathrow Express train, which takes approximately eight minutes." From BAA website

Terminal 1 & 3 其實全部係機場中心,可以行到.由terminal 1 出嚟跟著去underground station/ central bus station 嘅 sign, 落到去隨道就有Terminal 3 嘅 sign 沿著行就到. 我不清楚你的腳程快慢 行李多寡, 預30分鍾到
Airport terminal Map link: http://www.heathrowairport.com/terminalmaps/index_flash.htm?airport=Heathrow