Athletics Ireland have the following link with directions to the venue. Basically you park in Tullamore Harriers stadium which is in the town a few 100m from the town centre:
A course map and timetable are also at above link. The first race U11 girls starts at 11.30am. Everyone participating should try to be there by 10.30am to give chance to walk the course, warmup, get race numbers etc..
The easiest way to get to Tullamore is to stay on the motorway until Kilbeggan and turn right there for Tullamore. Its only a 10 minute spin from there. When you get to the first roundabout enternig Tullamore, turn right (town centre) and at next roundabout turn left (town centre). This takes you past the GAA stadium and into the town centre. Go straight through the centre and take the right hand fork at the top of the town (signposted Birr I think). After 200 or 300m, the athletics stadium is on the left hand side – watch carefully for it as you can easily pass it without realising.