There are two platforms at Longford but I don't think that this is the source of the delay as the 1800 is usually at Edgeworthstown by the time the 1805 down train gets there.
I think it is just poor timetable design. The up-train has to cross with the 1805 at Edgeworthstown but the crossing delay is build into the up-train's timetable - it has 19 minutes to get from Longford to Edgeworthstown. So basically is the 1805 runs late at all, that delay gets added to the 1800.
The real problem is that there is 24 minutes between the up-train leaving Mullingar and the down-train doing the same but it is about a 10 minute run which is very tight for Irish Rail to manage.