As I told Treasa, I think most are wary of the UK, given their influence and the recent cock-up of the economy in the region. However, I also think that Philip has a soft spot for Ireland. He speaks Irish Gaelic (ILR 2 going on 3) and would often visit Ireland with his parents when he was young.
Whereas he sees many similarities between himself and Treasa, I think he's drawn to Siobhan's sunny disposition, which is likely the only one he would have truly known at first. Pip would have reached out to offer his condolences and his support to her husband, which may have been the catalyst to a deeper friendship between Philip and Siobhan.