Clontarf Castle is in the suburbs, the 130 bus stop stops outside it and will take at least 20 minutes to get into the city centre (depending on traffic). There is a DART (train) station in Clontarf but it's nearer to Fairview and it's a good distance from the castle. There are a few restaurants/cafés in Clontarf and a small theatre (Viking about Connolly's Pub).
If you are just staying the weekend, staying closer to the city centre would probably be a better plan.
Yes we found that out any hotels close can recommend ? Thank you
At this stage a lot of places in the centre will likely be booked up - it's a very busy time in Dublin. Your best option might be to put your dates into one of the booking sites and see what's still available within your budget then come back to the forum with a shortlist for advice.