Two of the most common prepositions of time are 'for' and 'since'. They both express something that started in the past and continues in the present, but we use them differently. That can frequently lead to confusion in everyday speech. Here is a helpful guide to help you avoid this confusion.
'For' is used in sentences where the speaker is specifying the duration or amount of time. It expresses how long a particular thing or situation has been going on or happening. It can be used with all verb tenses.
In the above sentence, you are stating that the amount of time you have been living in Dublin is 5 years.
However, it is important to note that with 'for', the amount of time in question does not need to be exact. You could, for example, say something like:
'Since' is used when specifying the starting point of something. It expresses duration up to the present, indicating that something began in the past and is unfinished. It is usually used with the Present Perfect or Present Perfect Continuous tenses.
In the above sentence, you are stating that you started living in Dublin in 2008 and you still live there.
'Since' can also be used in the structure 'it is a (period of time) since'.
So, in summary, 'for' is used to specify the duration or amount of time a particular situation or thing has been going on, and 'since' specifies the starting point of something, indicating that it began in the past and is unfinished. The basic formula for using both correctly is as follows:
For + a duration of time
Since + a point in time, until now.