we intend to visit the Guiness Brewery in May. Should we buy tickets in advance trough their site to avoid long lines or is it better to buy them on the spot ?
we were just there and had purchased the Dublin pass, so we avoided the line. We felt that the Dublin pass was worth the cost and it saved us time in lines several times. We purchased them ahead of time and picked them up at the airport when we arrived.
In May you may not find really long lines. During the season you would be fighting with lots of other toursts. The Dublin Pass may be of benefit, depending on how long you will be in Dublin and what you will see.