Some good things to consider getting to go along with Seamus are:
Dead Rider
Grave Spirit
Molly
Seamus Avatar
Jack Daw
Seamus is easy, yet tough, to play. His crew tends to fail miserably if it must spread out in any way, so some Strategies can be very, very difficult for him.
So, what he has to work with:
Belles are awesome. They can reliably get their lure spell off, can sometimes do some damage, and are not the easiest model to kill for their low cost. Use them to pull models out of position, or pull back models of yours that are in a bad spot.
Sybelle is another awesome model as she can actually do some real damage. The combo that is reliable is to companion your belles with sybelle, lure a model into range of sybelle, and then flurry. Sybelle can also use Call Belle from time to time, but it isn't the easiest cast.
Seamus, for all of his seeming awesomeness, is no where near invincible nor very good a killing. His best bet is to go avatar ASAP.
Seamus Avatar is full of awesome. He wrecks crews due to his insane Anathema terrifying ability that NO model can ignore (plus he makes EVERYTHING of yours on the table terrifying as well for extra fun against the living). He is also wicked, which is key. Get him in the middle of a lot of models, scare the crap out of them (terrifying -> 17 check), and then hit them for damage as they try to run due to wicked. If there are bodies around, throw them a stuff.
Dead Rider is good, and nearly an auto-include for seamus at 30+ points.
Jack Daw is nothing more than the biggest and best tarpit/headache model your opponent will have to face. He is VERY, VERY hard to kill (only magical damage or focused strikes even hit him), and if something does get through, just discard a card and he ignores it. His no cheating aura is really nasty too... coupled with Seamus Avatar terrifying... you'll just chase stuff to the edge of the table or beat them to death.
Grave Spirit is a good totem just to give Sybelle armor, or Dead Rider armor.
Molly, while terrible leading crews on her own, is a fantastic support unit and makes your belles faster and more effective. She can re-summon belles and sybelle much, much easier than Seamus. She can also summon the necrotic machine. And if someone has an awesome spell, she can copy it (friend or foe). She is not easy to learn though.
Hope that helps a little. A great resource is
http://pullmyfinger.wikispaces.com/. I suggest going their for further ideas!!
I should note that models that target WP will have a field day with Seamus Avatar. Increasing his very useful terrifying decreases his WP to near auto-fail against everything levels. Pandora loves this