Why is English so strange

Actually if you exclude Dublin, English started being spoken in Ireland 400 years ago; famously, Grace O'Malley (Gráinne Ní Mháille) and Queen Elizabeth spoke Latin when they met at the end of the 16th Century, because they didn't know each other's language.

If you go by that reason, however, Icelandic would be even stranger, being essentially isolated from Europe since its colonisation. On the other hand, English was all but isolated, and Middle English, the first understandable by a current speaker, emerged during the Norman period, where contacts with the mainland were at their peak.

The Celtic origin of "do" isn't widely accepted either, as it only appears to some extent in some Middle Welsh texts. I don't think Irish even has or had the concept of an auxiliary verb. "Linguists have provided a variety of reasons for this development, saying that the emphatic use of do sprang up from borrowing from the Celtic or French tongues. But, as Fisher makes clear, these theories are “generally ruled out” (269)" [source].