I use EasyVFR by PocketFMS on my iPad.
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Perfect coverage of Ireland, and the Frequent Flyer Membership includes unlimited electronic flight plan filing. Flight plans are instantly transmitted to into the ATC system.
You can use it on your Apple (iPad & iPhone), Android, Windows 7&8, Windows Phone & Apple Mac system.
As for needing an external gps, it depends. Wifi Only iPads (ones that do not have a 3G Sim card slot) need an external GPS, as they do not have one built in.
iPads with a 3G Sim Card slot, have built in GPS. Most people are happy with the built in GPS, but some get occasional drop outs from it, and choose to get an external GPS.
Most Android systems have GPS's built in already.