There have been fundamental changes to UK immigration with the introduction of the Points based system. You must ensure that you check the UK Border Agency website regularly in case of any further changes. Please note that, for applications made on or after 6 April 2013, visa application fees will change. Additionally, UKBA will be abolished and will become 2 separate departments within the 'Home Office'. International Student Support will update information and weblinks here as soon as possible but this will take time while we familiarise ourselves with the new regulations.
It is mandatory for all students coming to the UK for longer than 6 months to obtain Entry Clearance in their own country before travelling to the UK. For students coming to the UK for a course of less than 6 months, there is a Student Visitor category of visa which you must apply for as Entry Clearance if you are a visa national (or on arrival in the UK if you are not a visa national). Please note this visa cannot be extended and will not allow you to work. For more information on this as well as the Tier 4 (General) category of visa, please see the information below.
If you intend to travel to the UK via Dublin you will not pass through any UK immigration point and will therefore not be able to obtain Leave to Enter as a Student Visitor on arrival. Therefore, you must apply for Entry Clearance before leaving your own country even if your course is for less than 6 months.
You will also find a Quick Guide on the UKBA website which may help you decide which is the appropriate category for you.