Derek John,
Just a couple of points of clarification:
Seimhiu is applied to the initial consonant of verbs in the past tense, except for some irregular verbs, and the rule does not apply to verbs that begin with a vowel.
If a verb begins with a vowel, the past tense adds "D'" to the front of the verb, so ith "eat" becomes d'ith "ate". Also, seimhiu is only applied where possible, as some consonants cannot be lenited in modern Irish, so léigh "read" (present tense) is also léigh "read" (past tense). Isn't it interesting that the spellings are the same in present and past in both English and Irish?
Another example of the above is líon "fill"; past tense is líon "filled", so my presumption is that you would have to gather the tense from context.
These two examples rely on the fact that the consonant "L" has no lenited form in modern Irish. The same can be seen with nigh "wash", where nigh is also the past tense, "washed," because "N" has no lenited form. It does make it a little confusing. The comforting part is, only L, N, and R cannot be lenited, among consonants in Irish, so there are not so many words that are affected by this rule.
Dale D