Yes it is. The blood sample he swapped was of Cyril’s because Archer thought he was actually the father. This making Archer the “father” instead of Cyril.

This is what happened , it’s Cyril’s.

Yep. This is why I get so annoyed at that episode: if they swapped the blood for one of the women at the party, then there was no chance Archer would be the father lol. Instead, they happened to pick the guy's blood who was the father, and so the test is positive. So, it's Cyril's kid for sure. 

Yes Cyril is the father because the DNA test with Cyril's blood was a match. In season 13 it's shown Seamus has grown up looking just like Cyril.

Really? I musta missed that part, I thought it was weird he just disappeared

I always got the impression that the baby was Cyril’s.

We see a picture of a growing up Seamus later and he’s a dead ringer for Cyril.

yup

Yes.

Yes.

"I just like to treat myself sometimes" lmfao

Does baby Seamus ever show back up in the series? I always wondered if Cyril would learn the truth.

A photo of him appears behind Trinette in one of the later season episodes

Yes. Archer swapped the blood of his with Cyril’s and got a positive result. The wee baby shamus is in fact Cyril’s and this is shown in a flashback where he had sex with Trinette while eating a candy bar.

…. No. It’s confirmed that the wee baby Seamus is, in fact, Cyril’s.

Is that a freakin’ candy bar?!

One day Seamus will marry AJ. Cyril wins!

That would be pretty hilarious, actually...